THP 

1  1  IJL*f 


STUDY  OF  THE  GROWTH 
OP  CROPS 


UNIVERSITY  OF  CALIFORNIA 
AT   LOS  ANGELES 


< 


A 

»  '  I  .!> 

\J 


THE    SOIL 


FIRST  EDITION        .       .       .        March  1903. 
SECOND  EDITION     .  .        September  1908. 


Printed  in  Great  Britain. 


Photograph  showing  transition  from  Rock  into  Subsoil  and  Soil  by  Weathering 
(Hythe  Beds,  Great  Chart,  Kent). 


( Frontispiece. 


THE    SOIL 

AN    INTRODUCTION   TO   THE   SCIENTIFIC 
STUDY    OF   THE    GROWTH    OF    CROPS 


BY   A.   D.    HALL,   M.A.  (Oxon.) 

DIRECTOR  OF  THE  ROTHAMSTED  STATION 
(LAWES  AOHICI  I.TI-UAI.  TKPST) 

FOREIGN  MEMBER  OF  THE  ROYAI.  ACADEMV  OF  AGRICri.TURI 

OF   SWEDEN 


SECOND   EDITION,    REVISED    AND    ENLARGED 


NEW  YORK 
E.  P.  DUTTON  AND  COMPANY 

1908 


DEDICATED  TO 
THE  WORSHIPFUL  COMPANY  OF  GOLDSMITHS 

THE  FIRST  PUBLIC  BODY  IN  THIS  COUNTRY  TO 

CREATE  AN   ENDOWMENT  FOR  THE 

INVESTIGATION  OF  THE  SOIL 


INTRODUCTION 

THE  study  of  the  soil,  which  is  fundamental  in  any 
application  of  science  to  that  part  of  agriculture  which 
deals  with  the  growth  of  crops,  has  received  greatly 
increased  attention  during  the  past  few  years.  The 
crude  chemical  point  of  view,  which  in  the  main 
regarded  the  soil  as  a  nutritive  medium  for  the 
plant,  has  been  altogether  extended,  by  a  considera- 
°}  tion  of  the  soil  as  the  seat  of  a  number  of  physical 

-  processes    affecting    the    supply    of   heat    and    of   air 
0  and     water    to     the     plant,    and    again    as    a    com- 
plex   laboratory,    peopled    by    many    types    of   lower 

^  organisms,    whose    function    is    in    some    cases    indis- 
pensable, in  others  noxious,  to  the  higher  plants  with 
which  the  farmer  is  concerned.     These  three  kinds  of 
reaction — chemical,    physical,    and    biological — interact 
upon  one  another  and   upon  the  crop  in  many  ways ; 
they  are  affected  by,  and  serve  to  explain,  the  various 
j  tillage    operations    which    have    been    learnt    by    the 
t  accumulated  -experience    of   the    farming    community, 

-  and   the   hope  for  future   progress   lies   in  the   further 
3  adaptation    for    practical    ends  of   these    processes   at 
2  work  in  the  soil.      But  it  must  not  be  supposed  that 
t:  science  is  yet  in  a  position  to  reform  the  procedure  of 
<  farming,  or  even  to  effect  an  immediate  increase  in  the 

V 

310830 


vi  INTRODUCTION 

productivity  of  the  land  :  agriculture  is  the  oldest  and 
most  widespread  art  the  world  has  known,  the  applica- 
tion of  scientific  method  to  it  is  very  much  an  affair 
of  the  day  before  yesterday.  Nor  can  we  see  our  way 
to  any  radical  acceleration  of  the  turnover  of  agricultural 
operations  that  shall  be  economical ;  the  seasons  and  the 
vital  processes  of  the  living  organism  are  stubborn  facts, 
unshapable  as  yet  by  man  with  all  his  novel  powers. 
But  even  if  the  best  farming  practice  is  still  a  step 
beyond  its  complete  explanation  by  science,  yet  the 
most  practical  man  will  find  his  perception  stimulated 
and  his  power  of  dealing  with  an  emergency  quickened 
by  an  appreciation  of  the  reasons  underlying  the 
tradition  in  which  he  has  been  trained ;  and  such  an 
introduction  to  the  knowledge  of  the  soil  it  is  the  aim 
of  this  little  book  to  supply.  The  book  is  primarily 
intended  for  the  students  of  our  agricultural  colleges 
and  schools,  and  for  the  farmer  who  wishes  to  know 
something  about  the  materials  he  is  handling  day  by 
day.  While  a  certain  knowledge  of  chemistry  is 
assumed,  it  is  hoped  that  the  subject  is  so  treated  as 
to  be  intelligible  to  the  non-technical  reader  who  is 
without  this  preliminary  grounding.  Though  the  book 
is  in  no  sense  an .  exhaustive  treatise,  it  has  been  my 
desire  to  give  the  reader  an  outline  of  all  the  recent 
investigations  which  have  opened  up  so  many  soil 
problems  and  thrown  new  light  on  difficulties  that 
are  experienced  in  practice.  The  scope  of  the  book 
precludes  the  giving  of  references  and  authorities  for 
all  the  statements  which  are  made ;  but,  for  the  sake 
of  the  more  advanced  student,  a  bibliography  has 
been  appended,  which  will  take  him  to  the  original 
sources  and  give  him  the  means  of  learning  both 
sides  of  the  more  controversial  questions. 

The  same  reason — want  of  space — has  prevented  me 


INTRODUCTION  vii 

from  giving  an  adequate  justification  of  some  of  the 
points  of  view  indicated.  Any  worker  in  so  novel  and 
unsurveyed  a  field  as  the  study  of  the  soil  still  presents, 
must  arrive  at  certain  personal  conclusions,  and  I  have 
tried  to  steer  a  middle  course  between  an  over  insistence 
on  these  points  on  the  one  hand,  and  the  colourlessness 
that  would  come  from  their  entire  exclusion  on  the 
other.  No  great  part  of  a  text-book  can  pretend  to 
be  original,  but  in  the  sections  dealing  with  the 
chemical  analysis  and  the  physics  of  the  soil,  I  have 
incorporated  a  good  many  unpublished  measurements 
and  observations ;  for  the  mass  of  the  results  on  which 
the  book  is  based,  I  am  chiefly  indebted  to  the  work 
of  Lawes  and  Gilbert,  as  set  out  in  the  Rothamsted 
Memoirs,  and  to  the  writings  of  Warington  in  this 
country,  of  King,  Hilgard,  and  Whitney  in  America, 
of  Wollny  in  Munich. 

I  have  to  thank  Professor  J.  Percival,  of  the  South- 
Eastern  Agricultural  College,  for  notes  respecting  the 
association  of  plants  with  specific  soils,  and  many 
suggestions  on  biological  questions ;  Major  H anbury 
Brown,  C.M.G.,  head  of  the  Egyptian  Irrigation 
Department,  for  information  concerning  "  salted  "  lands 
in  Egypt ;  Mr  F.  J.  Flymen,  who  has  been  associated 
with  me  in  carrying  out  a  soil  survey  of  the  counties 
of  Kent  and  Surrey,  and  has  executed  many  of  the 
observations  recorded  here ;  Mr  W.  H.  Aston,  one  of 
my  pupils,  to  whom  I  owe  the  observations  on  p.  135  ; 
and  finally,  Dr  J.  A.  Voelcker,  to  whom  I  am  greatly 
indebted  for  reading  the  proof-sheets,  and  making 
many  valuable  suggestions  thereon. 

A.  D.  HALL. 

HARPENDEN,  December  1902. 


PREFACE  TO  THE  SECOND  EDITION 

A  CONSIDERABLE  number  of  additions  and  alterations 
have  been  incorporated  in  the  present  edition.  These 
include  a  revision  of  the  method  recommended  for  the 
mechanical  analysis  of  soils,  the  method  now  given 
being  that  adopted  by  the  members  of  the  Agricultural 
Education  Association  in  this  country.  Owing  to  re- 
searches which  have  appeared  since  the  publication  of 
the  first  edition,  I  have  greatly  modified  the  views  I 
then  expressed  on  the  nature  of  clay,  and  on  the  part 
played  by  zeolitic  silicates  in  the  retention  of  ammonium 
and  other  salts  by  the  soil.  During  the  last  six  years, 
however,  the  greatest  additions  to  our  knowledge  of  the 
soil  are  those  dealing  with  bacteria ;  in  consequence, 
the  chapter  on  the  living  organisms  of  the  soil  has  been 
largely  rewritten  and  added  to.  A  number  of  minor 
corrections  have  been  made  in  the  text,  some  of  which 
represent  the  removal  of  errors,  and  others  modifications 
due  to  more  recent  research.  For  the  mistakes  which 
must  still  remain,  and  which  will  become  evident  in  the 
course  of  time,  I  must  ask  my  readers'  pardon  before- 
hand ;  in  dealing  with  so  complex  a  subject  as  the  soil 
we  are  still  far  from  final  conclusions,  many  of  our  most 
trusted  conclusions  are  only  rough  approximations  to 
the  truth,  and  by  the  progress  of  research  they  may  at 
any  time  require  remodelling  until  they  are  hardly 
recognisable. 

A.  D.  HALL. 

THE   ROTHAMSTED  EXPERIMENTAL   STATION, 
May  1908. 


CONTENTS 


INTRODUCTORY 

PAGE 

In  the  Scientific  Study  of  Soils,  Chemical,  Physical,  and  Bio- 
logical Considerations  are  involved  .  .  I 

CHAPTER  I 

THE  ORIGIN  OF  SOILS 

Sedentary  Soils,  and  Soils  of  Transport  —  Weathering  — 
The  Composition  of  Rock-forming  Minerals  and  their 
Weathered  Products — Distinction  between  Soil  and  Sub- 
Soil — General  Classification  of  Soils  .  6 


CHAPTER  II 

THE   MECHANICAL   ANALYSIS  OF   SOILS 

Nature  of  Soil  Constituents :  Sand,  Clay,  Chalk,  and 
Humus  —  Methods  of  Sampling  Soils  —  Methods  for 
the  Mechanical  Analysis  of  a  Soil — Interpretation  of 
Results  .  .  .  .  .  .  -32 

CHAPTER  III 

THE  TEXTURE   OF  THE   SOIL 

Meaning  of  Texture  and  Conditions  by  which  it  is  affected 
— Pore  Space  and  Density  of  Soils — Capacity  of  the  Soil 
for  Water — Surface  Tension  and  Capillarity — Percola- 
tion and  Drainage — Hygroscopic  Moisture  .  .       60 
xi 


xii  CONTENTS 

CHAPTER  IV 

TILLAGE  AND  THE  MOVEMENTS  OF  SOIL  WATER 

PAM 

Water  required  for  the  Growth  of  Crops  — The  Effect  of 
Drainage — Effects  of  Autumn  and  Spring  Cultivation, 
Hoeing  and  Mulching,  Rolling,  upon  the  Water  Con- 
tent of  the  Soil— The  Drying  Effect  of  Crops— Bare 
Fallows— Effect  of  Dung  on  the  Retention  of  Water  by 
the  Soil  .......  89 

CHAPTER  V 

THE  TEMPERATURE  OF  THE  SOIL 

Causes  affecting  the  Temperature  of  the  Soil — Variation 
of  Temperature  with  Depth,  Season,  etc. — Tempera- 
tures required  for  Growth — Radiation — Effect  of  Colour 
— Specific  Heat  of  Soils — Heat  required  for  Evapora- 
tion— Effect  of  Situation  and  Exposure — Early  and  Late 
Soils.  .  .  .  .  .  .  .120 

CHAPTER  VI 

THE  CHEMICAL  ANALYSIS  OF  SOILS 

Necessary  Conventions  as  to  the  Material  to  be  Analysed 
— Methods  Adopted — Interpretation  of  Results — Dis- 
tinction between  Dormant  and  Available  Plant  Food 
— Analysis  of  the  Soil  by  the  Plant — Determination 
of  "Available"  .Phosphoric  Acid  and  Potash  by  the 
Use  of  Weak  Acid  Solvents  .  .  .  .  139 

CHAPTER  VII 

THE  LIVING  ORGANISMS  OF  THE  SOIL 

Decay  and  Humification  of  Organic  Matter  in  the  Soil — 
Alinit — The  Fixation  of  Free  Nitrogen  by  Bacteria 
living  in  Symbiosis  with  Leguminous  Plants  —  Soil 
Inoculation  with  Nodule  Organisms— Fixation  of  Nitrogen 
by  Bacteria  living  free  in  the  Soil — Nitrification — 
Denitrification — Iron  Bacteria — Fungi  of  Importance 
in  the  Soil :  Mycorhiza,  and  the  Slime  Fungus  of 
"Finger-and-Toe" 168 


CONTENTS  xiii 

CHAPTER  VIII 

THE   POWER  OK  THE  SOIL  TO  ABSORB  SALTS 

I'AOI 

Retention  of  Manures  by  the  Soil — The  Absorption  of 
Ammonia  and  its  Salts ;  of  Potash  ;  of  Phosphoric 
Acid — Chemical  and  Physical  Agencies  at  Work — 
The  Non-Retention  of  Nitrates — The  Composition  of 
Drainage  Waters — Loss  of  Nitrates  by  the  Land — 
Time  of  Application  of  Manures  .  .  .  .211 

CHAPTER  IX 

CAUSES  OF   FERTILITY  AND  b^IUILITY  OF  SOILS 

Meaning  of  Fertility  and  Condition — Causes  of  Sterility : 
Drought,  Waterlogging,  Presence  of  Injurious  Salts — 
Alkali  Soils  and  Irrigation  Water — Effect  of  Fertilisers 
upon  the  Texture  of  the  Soil — The  Amelioration  of 
Soils  by  Liming,  Marling,  Claying,  Paring  and  Burn- 
ing— The  Reclamation  of  Peat  Bogs  .  .  .  233 

CHAPTER  X 

SOIL  TYPES 

Classification  of  Soils  according  to  their  Physical  or 
Chemical  Nature — Geological  Origin  the  Basis  of 
Classification  —  Vegetation  Characteristic  of  Various 
Soil  Types  :  Physical  Structure,  Chemical  Composition, 
Natural  Flora  and  Weeds  characteristic  of  Sands, 
Loams,  Calcareous  Soils,  Clays,  Peat,  Marsh,  and  Salt 
Soils — Soil  Surveys,  their  Execution  and  Application  .  271 

APPENDICES 

APPENDIX    I. — Chemical  Analyses  of  Typical  Soils  .          300,  301 
„  II. — Bibliography.  ....       302 

INDEX 


LIST  OF  ILLUSTRATIONS 


Photograph  showing  transition  from  Rock  into  Subsoil 
and  Soil  by  Weathering  (Hythe  Beds,  Great  Chart, 
Kent)  ......  Frontispiece 

1.  Photograph  of  Soil-sampling  Tools  .  .  .49 

2.  Diagram  illustrating  Pore  Space  .  .  .  .62 

3.  Diagram  illustrating  Capillary  Rise  and  Depression  of 

Liquids   .......        72 

4.  Photograph  illustrating  Liquid  Film  round  Soil  Particles        73 

5.  Diagram  illustrating  Liquid  Film  round  Soil  Particles  .         75 

6.  Water  Content  of  Columns  of  wetted  but  thoroughly 

drained  Sand  and  Soil  .....        76 

7.  Diagram  showing  Rainfall  and  Percolation  at  Rotham- 

sted,  1870-1905  ......        78 

8.  Soil  Temperatures  at  9  A.M.,  Monthly  Means      .  .122 

9.  Soil  Temperatures  at  9  A.M.,  Daily  Readings      .  .       123 

10.  Effect  of  Nature  of  Surface  upon  Soil  Temperatures       .       128 

1 1.  Temperatures  of  Drained  and  Undrained  Bog    .  .132 

12.  Distribution   of   the    Sun's    Rays    on    Southerly    and 

Northerly  Slopes  .  .  .  .  133 

13    Temperatures  (Max.  and  M in.)  at  Various  Altitudes       .       135 

14.  Nitrates  in  Cultivated  and  Uncultivated  Soil       .  .       195 

15.  Losses  of    Nitrogen    in    Drainage    from    Rothamsted 

Wheat  Plots       ...  .  226 

16.  Nature  and  Distribution  of  Alkali  Salts    .  .  .       246 


THE    SOIL 


INTRODUCTORY 

In  the  Scientific  Study  of  Soils,  Chemical,  Physical,  and 
Biological  Considerations  are  involved. 

THE  whole  business  of  agriculture  is  founded  upon  the 
soil ;  for  the  soil  the  farmer  pays  rent,  and  upon  his 
skill  in  making  use  of  its  inherent  capacities  depends 
the  return  he  gets  for  his  crops.  Taking  rent  as  a 
rough  measure  of  the  productive  value  of  land,  it  is 
clear  that  enormous  differences  must  exist  in  the  nature 
of  the  soil,  for  in  the  same  district  some  land  may  be 
rented  at  £2,  and  other  land  at  as  little  as  55.  per  acre. 
Of  course  rent  is  not  wholly  determined  by  the  nature 
of  the  soil,  but  depends  also  on  the  proximity  of  a 
market,  and  the  adaptability  of  the  land  to  special 
purposes ;  a  light  sandy  or  gravelly  soil,  almost  worth- 
less for  general  agricultural  purposes,  may  be  valuable 
in  the  neighbourhood  of  a  large  town,  because  its  earli- 
ness  and  responsiveness  to  manure  make  it  specially 
suitable  for  market  gardening. 

In  some  cases  the  difference  between  soils  is  seen 
in  the  quality  of  the  crop  produced  rather  than  in  the 
productiveness  ;  for  example,  the  "  red  lands  "  of  Dunbar 
are  famous  for  the  high  quality  of  the  potatoes 
grown  upon  them  :  such  potatoes  will  sell  at  8os.  to  905. 

A 


2  INTRODUCTORY 

per  ton,  when  potatoes  grown  upon  the  Lincoln  warp 
soils  are  at  6os.,  and  those  from  the  black  soils  of 
the  fen  country  are  only  fetching  453.  to  503.  This 
extra  price  for  the  red  land  potatoes  is  due  to 
the  fact  that  they  can  be  cooked  a  second  time, 
after  cooling,  without  changing  colour,  whereas  the 
ordinary  potato  is  apt  to  blacken  a  little  when  once 
cooked  and  allowed  to  grow  cold. 

The  scientific  study  of  soils  is  concerned  with  the 
differences  indicated  above ;  its  endeavour  is  to  obtain 
such  a  knowledge  of  the  constitution  of  the  soil  and  the 
part  it  plays  in  the  nutrition  of  the  plant,  as  will  make 
clear  the  cause  of  the  inferiority  of  any  given  piece  of 
land,  and  ultimately  enable  the  farmer  to  correct  it. 
The  problems  involved  are  far  more  complex  than 
they  appear ;  at  first  sight  nothing  would  seem  easier 
than  to  make  a  chemical  analysis  of  the  soil  and  find 
out  in  what  respects  it  differs  from  another  soil  of 
known  value ;  then  the  deficiencies  or  the  excesses,  as 
compared  with  the  good  soil,  could  be  corrected  by  suit- 
able manuring.  The  matter  is  not,  however,  quite  so 
simple,  for  if  on  the  one  hand  the  soil  can  be  considered 
as  a  great  reservoir  of  plant  food  which  can  be  recovered 
in  crops,  on  the  other  hand  it  is  equally  correct  to 
regard  the  soil  as  a  manufactory,  a  medium  for  trans- 
forming raw  material  in  the  shape  of  manure  into 
the  finished  article — the  crop.  In  new  countries  where 
virgin  soil  is  being  exploited,  and  in  districts  where  the 
systems  of  agriculture  are  primitive,  the  former  point  of 
view  is  the  correct  one ;  nothing  is  given  to  the  soil 
beyond  that  amount  of  labour  which  will  enable  some 
of  its  inherent  value  to  be  realised  in  a  crop.  Little  by 
little  the  capital,  which  may  be  practically  boundless, 
as  in  the  great  wheat  lands  of  Manitoba,  or  initially 
little  enough,  as  on  a  Connemara  heath,  is  being  drawn 


INTRODUCTORY  3 

upon  and  not  replaced.  But  in  a  Kentish  hop-garden 
or  other  land  where  an  intensive  system  of  cultivation 
is  practised,  the  crop  does  not  remove  as  much  as  it 
receives  ;  often  the  land  is  intrinsically  poor,  and  owes 
its  value  to  the  manner  in  which  it  will  elaborate 
the  raw  material  supplied  as  manure.  And  not  only 
are  these  very  special  soils  gaining,  rather  than  losing 
fertility  with  each  crop,  but,  from  a  general  point 
of  view,  all  countries  that  are  being  highly  farmed, 
like  parts  of  Great  Britain,  are  steadily  increasing  in 
fertility  at  the  expense  of  other  countries  which  are 
growing  crops  on  virgin  soil  ;  in  the  linseed,  the  maize, 
the  cotton  seed,  that  are  fed  to  our  stock,  there  travels 
to  our  soil  some  of  the  wealth  of  the  lands  upon  which 
these  crops  were  grown.  Hence  the  study  of  the 
inherent  resources  of  the  soil  is  perhaps  less  important 
than  an  examination  of  the  manner  in  which  the  soil 
deals  with  such  materials  supplied  under  cultivation. 

The  complete  knowledge  of  the  soil  and  the  part  it 
plays  in  the  nutrition  of  the  plant  requires  investigation 
along  three  lines,  which  may  be  roughly  classed  as 
—chemical,  physical  or  mechanical,  and  biological ; 
naturally  these  points  of  view  are  not  independent  of 
one  another,  but  are  only  so  separated  for  convenience 
of  study. 

In  the  first  place,  we  know  that  the  plant  derives 
certain  substances  necessary  to  its  development  from 
the  soil :  nitrogen  and  all  the  ash  constituents  reach 
the  plant  in  this  manner.  We  have,  therefore,  to 
investigate  the  proportions  in  which  these  constituents 
are  present  in  the  soil,  the  state  of  combination  in 
which  they  may  respectively  exist,  and  the  variations 
in  these  factors  normally  exhibited  by  typical  soils, 
all  of  which  questions  may  be  described  under  the 
head  of  chemical  analysis.  Further  investigations  of  a 


4  INTRODUCTORY 

chemical  nature  deal  with  the  power  of  various  soils  to 
retain  manure,  the  causes  of  sterility  or  fertility,  and 
the  measures  that  can  be  adopted  for  the  amelioration 
of  soils. 

The  soil  is,  however,  not  merely  a  storehouse  of  food 
for  the  plant,  since  water  is  equally  indispensable  to 
its  existence,  and  is  immediately  derived  from  the  soil ; 
hence  it  is  of  prime  importance  to  study  the  causes 
which  underlie  the  movement  of  water  in  the  land, 
and  its  supply  to  the  growing  crop.  In  the  relation 
between  soil  and  water  the  cultivation  to  which  the 
land  is  subjected  plays  a  prime  part,  hence  it  will  be 
necessary  to  trace  the  effect  of  each  of  the  main  opera- 
tions of  tillage  upon  the  structure  of  the  soil.  Again, 
the  texture  of  the  soil  and  the  proportions  of  water 
and  air  it  retains,  affect  its  temperature  and  that 
responsiveness  to  change  of  season  which  we  roughly 
indicate  by  the  terms  "early"  and  "late"  soils.  The 
general  consideration  of  these  questions  may  be  termed 
soil  physics. 

Finally,  the  soil  is  not  a  dead  mass,  receiving  on 
the  one  hand  manure,  which  it  yields  again  to  the 
crop  by  purely  mechanical  or  chemical  processes ;  it 
is  rather  a  busy  and  complex  laboratory  where  a 
multitude  of  minute  organisms  are  always  at  work. 
By  the  action  of  some  of  these  organisms,  vegetable 
residues  and  manures  are  reduced,  we  might  almost  say 
digested,  to  a  condition  in  which  they  will  serve  as  food 
for  plants  ;  others  are  capable  of  bringing  into  combina- 
tion, or  "  fixing,"  the  free  nitrogen  gas  of  the  atmosphere, 
and  therefore  add  directly  to  the  capital  of  the  soil ; 
others  again  are  noxious  or  destructive  to  the  food 
stores  in  the  soil. 

The  work  of  these  organisms  is  much  affected  by 
cultivation  ;  in  fact,  it  would  not  be  too  much  to  say 


INTRODUCTORY  5 

that  most  of  the  operations  upon  the  farm  have  received 
a  new  light  from  the  knowledge  that  has  been  acquired 
in  the  last  few  years  of  the  living  processes  taking  place 
in  the  soil.  In  this  direction  also  new  developments 
of  agriculture  seem  to  be  possible,  and  though  the 
progress  is  only  small  as  yet,  we  see  indications  that 
the  productive  capacity  of  the  land  may  be  per- 
manently increased  by  the  introduction  of  certain 
organisms  capable  of  assisting  the  work  of  the  higher 
plants. 

On  the  biological  side  we  have  also  to  study  the 
association  of  certain  plants  with  particular  soils ;  an 
examination  of  the  natural  flora  of  any  district  will 
show  that  some  species  are  almost  confined  to  sandy 
soils,  others  to  soils  containing  chalk,  rarely  wandering 
on  to  different  types  of  soil ;  again,  particular  weeds 
are  characteristic  of  clay  land,  others  of  sand ;  and 
some  even  of  our  cultivated  crops  show  a  marked 
intolerance  for  particular  soils. 

The  full  story  of  the  soil  cannot  yet  be  told ;  small 
wonder  that  in  the  course  of  the  many  centuries  man 
has  been  cultivating  the  face  of  the  earth,  he  has  found 
out  much  which  science  can  barely  explain,  still  less 
improve  upon.  Nor  are  the  problems  simple — the 
food,  the  water,  the  temperature,  the  living  organisms 
in  the  soil  are  all  variables,  affected  by  cultivation  and 
climate,  themselves  also  variable ;  they  all  act  and 
react  upon  one  another  and  upon  the  crops ;  hence  we 
can  easily  understand  that  the  smallest  farm  may 
present  problems  beyond  the  furthest  stretch  of  our 
knowledge. 


CHAPTER  I 

THE  ORIGIN   OF  SOILS 

Sedentary  Soils,  and  Soils  of  Transport — Weathering — The  Com- 
position of  Rock-forming  Minerals  and  their  Weathered 
Products — Distinction  between  Soil  and  Subsoil — General 
Classification  of  Soils. 

THE  study  of  soils  must  begin  with  some  knowledge 
of  their  origin  and  their  relationship  to  the  rocks  that 
underlie  them,  out  of  which,  in  most  cases,  they  have 
been  formed. 

Perhaps  the  best  way  of  arriving  at  an  idea  of  the 
natural  processes  which  result  in  soil,  is  to  visit  a 
river  valley  and  examine,  first  a  quarry  on  the  flanks 
of  the  hills,  and  then  one  of  the  cuttings  for  gravel  or 
brick  earth,  which  often  lie  a  little  above  the  river  level. 

The  face  of  the  quarry  shows  at  a  depth  of  10  feet 
or  so  from  the  surface  the  massive  rock,  unaltered  as 
yet  by  any  action  of  the  weather.  Closer  examination, 
however,  shows  that  even  at  this  depth  the  rock  is 
not  quite  solid  ;  if  it  be  a  stratified  rock  the  planes  of 
bedding  are  apparent,  along  which  the  rock  can  be 
split  Joints  again  traverse  the  rock  at  right  angles 
to  the  bedding  planes,  and  along  both  joints  and  bed- 
ding planes  it  is  evident  that  water  makes  its  way,  for 
the  edges  of  the  cracks  are  slightly  altered  and  dis- 
coloured. Nearer  the  surface,  the  cracks  and  lines  of 


CHAP.  I.]  SEDENTARY  SOILS  7 

weakness  in  the  rock  become  more  palpable ;  in  some 
cases  the  joints  have  been  forced  open  by  the  intrusion 
of  the  roots  of  trees ;  minor  cracks  have  started  from 
the  main  ones,  and  the  disintegration  of  the  rock  at 
the  edges  of  the  cracks  has  proceeded  further,  till  at  a 
distance  of  3  or  4  feet  from  the  surface  the  whole 
material  is  loose  and  shattery,  though  still  preserving 
the  appearance  of  solid  rock.  Still  nearer  the  surface, 
the  rock  structure  seems  to  have  disappeared ;  rock 
may  be  there  in  lumps  and  fragments,  but  it  is  em- 
bedded in  small  material  that  may  fairly  be  termed  soil 
or  earth.  Still  nearer  the  surface  the  rock  fragments 
become  smaller,  and  the  proportion  of  fine  earth  larger, 
till  in  the  top  9  inches  or  so  a  new  change  begins. 
Here  the  stones  are  generally  small,  and  the  material 
is  dark  from  the  admixture  of  decaying  vegetable 
matter,  residues  of  the  crops  that  have  covered  the 
surface  for  long  ages.  This  is  the  soil  proper,  generally 
shading  gradually  into  the  subsoil  below,  which  in  its 
turn  passes  insensibly  into  the  underlying  rock.  It  is 
obvious  that  a  soil  such  as  we  have  been  describing  has 
been  directly  formed  from  the  rock — it  is,  in  fact,  the 
rock  disintegrated  and  reduced  by  frost  and  snow,  air 
and  rain ;  all  those  agencies  we  group  together  under 
the  name  of  "weathering."  We  are  dealing  with  a 
soil  formed  in  situ,  or,  as  it  is  sometimes  termed,  a 
sedentary  soil. 

The  frontispiece  shows  a  photograph  of  such  a  case 
of  weathering  of  rock  into  subsoil  and  soil,  as  seen  in  a 
section  of  the  Hythe  Beds,  near  Great  Chart,  Kent. 

But  when  we  examine  the  section  of  the  gravel  pit 
or  the  brick  earth  workings  lower  down  in  the  valley, 
the  sequence  is  not  the  same ;  we  still  have  the  soil 
proper  passing  into  the  subsoil,  but  this  is  fairly  uniform 
throughout  instead  of  showing  a  progressive  change 


8  THE  ORIGIN  OF  SOILS  [CHAP. 

as  we  descend ;  if  it  be  gravel,  the  stones  continue  of 
the  same  size ;  if  brick  earth,  neither  stones  nor  hard 
stratified  clay  make  their  appearance.  Should  the 
exposed  section  be  deep  enough,  we  find  at  last  the 
subsoil  suddenly  giving  place  to  entirely  different 
material — solid  chalk,  or  massive  clay,  or  sandstone,  as 
the  case  may  be — perhaps  incapable,  when  disintegrated, 
of  furnishing  the  stuff  of  which  the  upper  stratum  of 
gravel  or  brick  earth  is  composed.  In  this  upper 
stratum  we  see  the  clearest  evidence  of  the  action  of 
water ;  the  brick  earth  is  free  from  stones  and  is  of  even 
texture,  the  gravel  contains  hardly  any  fine  material, 
and  its  constituent  stones  are  worn  and  partly  rounded  ; 
only  running  water  can  thus  sift  the  heterogeneous 
results  of  the  weathering  of  rocks,  and  grade  them  into 
different  deposits.  From  what  can  be  seen  of  the 
present  work  of  the  river,  it  is  clear  that  the  brick  earth 
was  deposited  where  the  water  was  moving  very  slowly, 
in  quiet  bays  and  in  cut-offs,  which  only  from  time  to 
time  get  filled  up  with  muddy  flood  water ;  the  gravel 
must  have  been  laid  down  in  the  strongest  wash  of  the 
currents. 

Soils  and  subsoils  of  this  type,  which  bear  no 
particular  relation  to  the  underlying  rocks,  but  have 
travelled  from  a  distance  by  means  of  running  water 
or  some  kindred  agency,  are  known  as  soils  of  transport, 
or,  to  use  the  terminology  of  the  Geological  Survey, 
as  drift  soils. 

Weathering, 

The  study  of  geology  teaches  us  that  nearly  all  the 
rocks  termed  stratified  or  sedimentary,  which  cover 
the  greater  part  of  the  surface  of  the  British  Islands, 
have  been  formed  from  the  waste  of  previous  rocks  by 
weathering,  and  by  the  subsequent  redeposit  and  con- 


I.]  WEATHERING  9 

solidation  of  the  weathered  material.  A  grain  of  sand, 
for  example,  is  practically  indestructible  ;  it  may  have 
become  cemented  to  the  other  grains  on  the  sea  beach 
where  it  was  lying,  and  give  rise  to  the  rock  we  term 
sandstone  ;  the  rock  thus  formed  may  have  been  elevated 
into  dry  land,  broken  up  into  loose  grains,  and  washed 
down  to  the  sea  to  form  a  new  beach,  over  and  over  again 
in  the  world's  history ;  so  long  a  time  has  elapsed  since 
water  first  began  to  work  on  the  earliest  rocks.  For  this 
reason,  if  we  want  to  trace  out  the  origin  of  a  soil  in 
detail,  we  must  in  most  cases  go  beyond  the  sedimen- 
tary rock  from  which  it  immediately  derives,  back  to  the 
so-called  primitive  or  crystalline  rocks,  which  represent 
in  a  sense  the  original  materials  of  the  earth's  crust. 

Here  we  shall  find  certain  fundamental  minerals, 
which  in  a  weathered  state,  altered  both  mechanically 
and  chemically,  go  to  form  both  the  sedimentary  rocks 
and  the  soil  which  is  our  immediate  study.  Though 
the  number  of  distinct  minerals  is  immense,  practically 
the  mass  of  the  earth's  crust  is  made  up  of  a  few  only ; 
silica,  various  complex  silicates  of  alumina,  iron,  lime, 
magnesia,  potash,  and  soda,  together  with  carbonate  of 
lime,  which  is  generally  of  organic  origin,  are  all  that 
need  be  considered  in  relation  to  soils. 

The  various  agencies  which  reduce  rocks  to  soil, 
grouped  under  the  general  term  of  weathering,  may  be 
distinguished  as  mechanical — including  the  work  of  alter- 
nations of  temperature,  frost,  wind,  rain,  and  glacial  ice — 
and  chemical,  the  complex  effects  of  solution  and  oxida- 
tion that  are  brought  about  by  water,  especially  when 
charged  with  carbonic  acid. 

In  dry  climates  the  alternations  of  temperature 
between  day  and  night  set  up  sufficient  strain  to  fracture 
even  large  rocks,  and  eventually  reduce  them  to  dust. 
The  dust  and  sand  of  the  deserts  of  Central  Asia,  the 


io  THE  ORIGIN  OF  SOILS  [CHAP. 

barren  lands  of  the  United  States,  and  many  parts  of 
both  North  and  South  Africa,  are  formed  in  this  way ; 
because  of  the  dryness  of  the  atmosphere,  radiation  is 
extreme,  and  the  temperature  of  the  rock  surface  will 
rise  to  60°  C.  in  the  day  and  fall  below  zero  at  night 
Crystalline  rocks  soon  disintegrate  under  such  alterna- 
tions of  temperature,  and  the  fine  angular  dust  thus 
formed  is  transported  by  wind  into  the  plains  and  valleys, 
giving  rise  to  soils  largely  wind-borne.  Richthoven  has 
supposed  that  the  immense  loess  deposits  of  China  are  in 
the  main  dust  that  has  been  blown  from  the  Central 
Asian  deserts.  Even  in  a  humid  country  like  our  own  the 
wind  plays  a  considerable  part  in  forming  soil,  material 
being  constantly  removed  from  any  bare  surface  and 
deposited  elsewhere  as  dust  When  all  the  country 
was  in  its  natural  state  and  clothed  with  vegetation, 
the  amount  of  transport  as  dust  must  have  been  con- 
siderably smaller  than  at  present,  but  even  then  worm 
casts  brought  up  in  the  spring  would  crumble  in  dry 
weather,  and  be  moved  to  lower  levels  by  the  wind. 
The  thickness  of  the  dust  deposit  may  be  gauged 
by  the  rapidity  with  which  shingle  beds  newly  won 
from  the  sea  become  covered  with  vegetation ;  in 
the  neighbourhood  of  Dungeness  shingle  beds  known 
to  be  less  than  fifty  years  old  are  already  clothed 
with  a  scanty  flora.  On  scraping  away  a  few  inches 
of  the  shingle  the  interstices  between  the  stones  are 
found  to  be  filled  with  a  fine  black  sand,  which 
can  only  have  been  wind-borne ;  this  rapidly  increases 
as  the  first  vegetation  checks  the  velocity  of  the 
wind  above  the  stones  and  arrests  the  dust,  till  at  last 
it  reaches  the  surface  and  the  grass  begins  to  spread 
over  the  stones.  Exact  dates  are  difficult  to  obtain, 
but  probably  considerably  less  than  a  century  is  suffi- 
cient to  form  a  thin  turf  over  a  bare  shingle  bed. 


I.]  WEATHERING  \\ 

But  the  great  weathering  agency  in  temperate  climates 
is  undoubtedly  frost  acting  upon  water  contained  within 
the  rocks  and  stones ;  the  water  expands  as  it  changes 
into  ice,  and  exerts  an  enormous  pressure — indeed  about 
100  atmospheres  would  be  required  to  keep  water  in  a 
liquid  condition  at  —  i°  C.  All  rocks  when  freshly  exposed, 
hold,  by  capillary  attraction,  a  certain  amount  of  water 
known  as  the  "quarry  water,"  which  amounts  in  the 
white  chalk  to  as  much  as  19  per  cent  A  piece  of  such 
chalk  will  be  shattered  into  fragments  by  a  single 
night's  frost  Even  after  the  quarry  water  has  been 
dried  out  the  most  close-grained  rocks  will  absorb  a 
small  quantity  of  water.  The  face  of  polished  granite 
rapidly  deteriorates  in  severe  climates,  owing  to  the 
freezing  of  the  water  that  finds  its  way  into  the  minute 
divisions  between  the  crystals :  Cleopatra's  Needle, 
which  had  retained  its  smooth  face  for  centuries  in 
Egypt,  soon  became  affected  after  its  removal  to 
London,  and  has  to  be  protected  by  a  waterproof 
varnish,  as  have  all  the  granite  monuments  in  Canada. 

In  nature  also,  all  rocks  are  traversed  by  joints  and 
bedding  planes ;  these  cracks  are  filled  with  water  and 
opened  and  extended  by  its  conversion  into  ice  in  the 
winter,  till  finally  a  block  is  wedged  off  and  a  fresh 
surface  exposed  to  the  action.  Where  flagstones  are 
quarried,  the  workmen  are  in  the  habit  of  saturating  the 
surface  of  the  rock  with  water  before  the  winter  sets  in  : 
thus  the  rock  is  split  along  its  bedding  planes  more 
effectively  than  by  any  artificial  means.  The  fragments 
that  have  been  broken  off  the  main  rock  will  be  con- 
tinually reduced  in  size  by  successive  frosts,  until  they 
reach  the  ultimate  fragments  which  are  no  longer 
penetrated  by  water ;  even  in  a  soil  the  disintegration  is 
still  proceeding. 

The  weathering  agencies  just  described  would  gradu- 


i  a  THE  ORIGIN  OF  SOILS  [CHAP. 

ally  cover  any  exposed  rock  with  a  layer  of  debris, 
which  would  protect  the  lower  layers  from  further  action 
were  it  not  that  the  rain  is  always  washing  the  finer 
particles  into  the  valleys  and  so  leaving  the  rock  open  to 
fresh  attack.  Even  on  grass  land  the  fine  mould  brought 
to  the  surface  by  worms,  moles,  ants,  etc,  is  constantly 
travelling  downhill  by  the  agency  of  rain.  On  arable 
land  containing  stones  it  is  a  common  expression  to  say 
that  the  stones  "  grow  "  :  however  thoroughly  the  surface 
may  be  picked  clean  of  stones,  in  a  year  or  two  they  will 
seem  as  numerous  as  ever ;  the  fine  soil  gets  washed 
away  to  lower  levels,  leaving  the  stones  standing  upon 
the  surface.  Even  the  stones  themselves  gradually  creep 
downhill,  the  rain  undermines  them  till  they  fall  over, 
they  must  fall  a  little  lower  down  the  slope,  until  they 
eventually  reach  the  valley  and  are  subject  to  further 
transport  by  running  water.  At  the  bottom  of  many  of 
the  smaller  dry  valleys  on  the  chalk  rests  an  enormous 
accumulation  of  flints  of  all  sizes  ;  in  one  case  in  a  small 
upland  valley  the  deposit  was  6  or  7  feet  thick,  and  the 
unworn  flints  were  so  close  as  to  be  practically  in 
contact,  only  the  interstices  being  occupied  by  soil ;  yet 
the  surface  carried,  good  crops. 

The  material  which  thus  creeps  down  the  sides  of 
the  valleys  is  further  sorted  out  by  the  streams  and 
rivers  and  deposited  as  beds  of  gravel,  sand,  or  clay, 
the  "  alluvium  "  which  underlies  the  level  river  meadows. 
The  coarser  the  material  the  more  readily  will  it 
settle,  the  finer  particles  are  only  deposited  when  the 
velocity  of  the  stream  has  been  almost  entirely  checked. 
The  gravel  and  sand  are  deposited  in  and  about  the 
stream  course  itself,  the  finer  material  falls  on  the  meadows 
in  flood  time,  so  that  their  level  is  gradually  raised  from 
year  to  year.  Wherever  the  meadows  get  water-logged 
the  surface  vegetation  will  begin  to  accumulate  as 


I.]  WEATHERING  13 

peat;  the  stream  also  wanders  about  from  side  to 
side  of  the  valley,  hence  borings  through  any  exten- 
sive deposit  of  alluvium  will  disclose  alternating  beds 
of  gravel,  sand,  brick  earth,  and  peat,  of  variable 
extent  and  thickness.  The  great  alluvial  flats  or 
marshes  at  the  mouths  of  many  of  our  rivers  are 
formed  in  this  manner;  the  deposit  takes  place  in  the 
sea  or  in  the  estuary,  until  the  tides  and  currents  work 
the  material  up  to  high-water  mark,  after  which  only 
fresh-water  beds  are  laid  down. 

Although  most  of  the  materials  of  which  rocks  are 
composed  are  in  the  ordinary  sense  insoluble  in  water, 
few  of  them,  except  the  pure  sand  grains,  can  resist  the 
attack  of  water  charged  with  carbonic  acid.  The  rain 
water  when  it  reaches  the  ground  has  little  carbonic  acid 
in  solution,  but  the  gases  in  the  soil  contain  a  consider- 
able quantity  derived  from  the  decay  of  vegetable  matter 
in  the  surface  layer,  and  the  water  in  contact  with  these 
gases  will  dissolve  a  proportionate  amount  The  pro- 
portion of  carbonic  acid  in  the  soil  gases  varies  very 
much  both  with  the  permeability  of  the  soil  and  the 
proportion  of  humus,  but  at  a  depth  of  1-5  metres 
Wollny  found  it  vary  from  3-84  per  cent  to  14-6  per 
cent  at  various  periods  of  the  year.  At  greater  depths 
the  amount  is  still  higher,  so  that  the  percolating  water 
becomes  a  weak  solution  of  carbonic  acid,  and  attains  a 
considerable  solvent  power.  Not  only  are  the  alkaline 
silicates  attacked  by  the  weak  acid  thus  formed,  but  as 
lime,  magnesia,  and  iron  protoxide  also  form  soluble 
bicarbonates,  all  minerals  containing  these  bases  are 
liable  to  attack.  Probably  some  of  the  organic  acids 
produced  by  the  decay  of  vegetable  matter  in  the  sur- 
face soil  aid  in  the  solvent  power  of  soil  water ;  yet, 
undoubtedly,  water  containing  carbonic  acid  is  the  great 
natural  solvent,  and  some  of  the  more  striking  cases  of 


1 4  THE  ORIGIN  OF  SOILS  [CHAP. 

its  action  in  breaking  down  rocks  will  be  discussed  later 
under  the  heads  of  felspar,  augite,  and  calcium  carbonate. 

The  attack  of  frost  and  water  upon  rocks  is  much 
assisted  by  the  roots  of  plants  and  trees ;  if  we  examine 
a  fresh  section  of  the  soil  over  a  quarry  or  brick  pit,  the 
roots  of  ordinary  field  plants  can  be  traced  downwards 
for  4  feet  or  more,  while  the  roots  of  a  tree  may  be  seen 
working  far  into  tiny  fissures  of  the  almost  unaltered 
rock.  The  roots  follow  the  water  in  the  fissures  :  at  first 
they  can  enter  very  minute  cracks ;  as  they  grow,  the 
pressure  they  exert  widens  the  cracks  ;  finally,  the  roots 
decay  and  leave  a  channel  down  which  water  can  per- 
colate freely.  The  fine  roots  themselves  have  a  certain 
solvent  action ;  after  plants  had  been  grown  in  a  pot 
filled  with  powdered  granite  rock,  which  had  been  freed 
from  all  fine  particles  by  washing,  an  appreciable  quantity 
of  mud  and  clay  was  found  to  have  been  formed. 

The  opening  up  of  the  subsoil  to  weathering  by  the 
action  of  roots  is  also  carried  out  by  worms,  which  have 
been  observed  making  their  burrows  to  the  depth  of  5 
feet,  thus  introducing  both  air  and  water  into  the  lower 
strata.  But  the  great  work  of  worms  in  regard  to  soil 
lies  rather  in  the  production  of  the  fine  surface  layer  of 
mould  rich  in  vegetable  matter :  Darwin  calculated  that 
on  an  ordinary  chalky  pasture  the  whole  of  the  fine 
surface  soil  to  a  depth  of  10  inches  was  passed  through 
worms  and  cast  up  on  the  surface  in  the  course  of  fifty 
years.  During  their  passage  through  the  gizzard  of  the 
worms  the  stony  particles  will  receive  a  certain  amount 
of  rubbing  and  be  reduced  in  size,  so  that  some  of  the 
finer  particles  in  the  soil  owe  their  origin  to  worms. 
The  deposit  of  the  fine  soil  on  the  surface  in  the  shape 
of  worm  casts,  which  are  afterwards  spread  by  the  action 
of  rain  and  wind,  explains  why  chalk,  ashes,  or  even 
stones  placed  on  pasture  land  gradually  sink  below  the 


I.]       TRANSPORT  OF  WEATHERED  MATERIAL       15 

surface.  Darwin  found  in  one  case  that  a  layer  of  burnt 
marl  spread  on  the  surface  had  sunk  3  inches  in  fifteen 
years,  in  another  case  a  layer  of  chalk  was  buried  7 
inches  after  an  interval  of  twenty-nine  years ;  in  neither 
case,  however,  can  we  estimate  the  part  played  by  the 
accretion  of  dust  in  forming  this  deposit.  When  we 
consider  for  how  long  a  period  worms  must  have  been 
working  in  our  cultivated  soils,  it  is  clear  that  the  whole 
must  have  been  through  them  over  and  over  again,  and 
that  much  of  the  fineness  of  the  surface  soil  must  be  due  to 
their  action,  both  in  actually  grinding  the  fragments  and 
in  constantly  bringing  the  finest  portions  back  to  the  top. 

In  addition  to  the  alluvial  deposits  proper,  which  are 
still  in  process  of  formation,  beds  of  gravel,  sand,  and 
brick  earth  occur  in  many  river  valleys,  as  terraces  on 
the  flanks  of  the  hills,  often  much  cut  and  denuded  by 
the  modern  river.  These  high  level  formations  prob- 
ably represent  alluvial  deposits  of  a  former  epoch 
where  the  general  slope  of  the  land  was  greater  and  the 
rivers,  fed  by  a  higher  rainfall  in  the  hills,  ran  in  greater 
volume.  That  the  material  of  which  these  deposits  con- 
sist has  been  sorted  by  running  water  is  evident  from 
the  uniformity  of  size  it  possesses  in  each  bed :  while 
the  coarseness  of  the  gravel,  and  the  fact  that  in  some 
cases  the  stones  are  not  made  from  the  immediately 
underlying  rock,  all  point  to  a  great  lapse  of  time  and  a 
river  of  higher  transporting  power  than  the  present  one. 
The  wide  deposits  of  brick  earth  in  the  neighbourhood 
of  London  and  in  East  Kent  were  probably  laid  down 
either  by  floods  on  the  river  meadows  or  in  quiet  bays 
and  lagoons  of  an  estuary. 

Over  a  great  part  of  Britain  north  of  the  Thames, 
especially  in  the  midlands  and  the  eastern  counties, 
the  surface  of  the  land  is  covered  with  beds  of  clay 
and  sand  which  owe  their  origin  to  glacial  ice.  In 


THE  ORIGIN  OF  SOILS 


[CHAP. 


Scotland,  the  north  of  England,  and  Wales,  these  beds 
are  full  of  ice-scratched  stones,  and  clearly  represent 
material  that  has  been  ground  down  by  a  moving 
glacier :  but  the  origin  of  the  glacial  drift  of  the  eastern 
counties  is  more  obscure,  for  water  seems  to  have  played 
some  part  in  its  formation.  The  beds  are  mostly  stiff 
and  clayey  in  character,  and  by  their  included  fragments 
show  from  what  formation,  as  a  rule  not  very  remote, 
they  have  been  derived. 


Rock-forming  Minerals. 

In  the  solid  crust  of  the  earth  D'Orbigny  has 
estimated  that  the  chief  minerals  are  present  in  the 
following  proportions — felspars,  48  per  cent. ;  quartz, 
35  per  cent. ;  micas,  8  per  cent. ;  talc,  5  per  cent. ; 
carbonates  of  lime  and  magnesia,  i  per  cent. ; 
hornblende,  augite,  etc.,  I  per  cent. ;  other  minerals 
and  weathered  products,  2  per  cent. 

The  following  table  shows  the  composition  of  these 
chief  minerals,  with  a  few  others  that  play  some  part  in 
the  formation  of  soil : — 


8 

1 

| 

8 

3 

•9 

I 
| 

§   0 

£2 

C  M 

|| 

1 

CO 

I 

a 

3 

^ 

So 

£o 

£ 

Quartz    . 

IOO 

(  Orthoclas 

e 

64-2 

17 

... 

18-4 

Felspar-!  Albite 
[Anorthite 

68-6 
43-1 

i'i-8 

20 

I9-6 
36-9 

45 

6 

o 

26 

i 

Mica        . 

to 

to 

to 

to 

to 

50 

IO 

1-5 

36 

4-7 

/  Hornblende  . 
\Augite  . 

39 
to 

49 

... 

IO 
to 

27 

IO 

to 

3 
to 
IS 

3 
to 
20 

Olivine    . 

. 

41 

... 

... 

49-2 

9-8 

Talc 

63-5 

31-7 

... 

4-8 

I.]  ROCK-FORMING  MINERALS  17 

Quartz,  the  crystalline  form  of  silica,  is  found 
massive  and  in  veins  in  the  primitive  rocks,  and  in 
fragments  of  all  sizes  in  the  granites,  gneisses,  and 
similar  rocks.  From  the  waste  of  these  crystalline 
rocks  are  derived  the  sandstones  of  all  geological  ages, 
and  directly  or  indirectly  the  sands  now  existing. 
In  a  sandstone  rock  the  grains  of  quartz  are  bound 
together  by  a  cement,  which  may  be  oxide  or  car- 
bonate of  iron,  as  in  the  Lower  Greensand  of  Surrey 
and  Beds,  and  in  some  of  the  Wealden  sandstones,  or 
carbonate  of  lime,  as  in  the  Kentish  Rag,  or  even  silica 
itself,  as  in  the  hard  blocks  of  tertiary  sandstone,  which 
are  left  as  "  grey  wethers  "  on  the  surface  of  the  chalk. 
In  some  of  the  older  sandstones  the  rock  is  practically 
homogeneous ;  heat,  pressure,  and  solution  having 
thoroughly  felted  the  grains  together.  Many  sand- 
stones weather  rapidly,  through  the  solution  of  the 
cement  binding  the  grains  together ;  the  resulting  sand 
has  the  same  texture  as  it  possessed  before  it  was 
cemented  into  a  rock. 

The  grains  of  sand  that  are  first  weathered  from  a 
crystalline  rock  possess  an  angular  shape,  but  are 
soon  rubbed  down  in  running  water  into  rounded 
grains  with  a  surface  like  fine  ground  glass.  Hence 
the  degree  of  angularity  which  the  sand  grains  show  gives 
some  indication  of  the  amount  of  wear  and  tear  they 
have  suffered  since  their  origin  as  sand.  Below  a  certain 
size,  however,  quartz  grains  seem  no  longer  capable  of 
rubbing  against  one  another,  but  remain  angular  even 
after  long  travel  in  running  water.  Daubree  has  shown 
that  angular  fragments  of  sand  of  less  than  01  mm. 
in  diameter  will  travel  in  water  without  becoming 
rounded,  hence  any  rounding  of  smaller  grains  of  sand 
must  have  been  due  to  solution. 

Silica  in  the  crystalline  state  is  very  slightly  soluble 

B 


1 8  THE  ORIGIN  OF  SOILS  [CHAP. 

in  water,  a  certain  amount  of  solution  taking  place 
even  at  ordinary  temperatures :  most  natural  waters 
show  a  little  silica  in  solution,  though  this  more 
probably  arises  from  the  decomposition  of  natural 
silicates  by  water  containing  carbonic  acid,  rather  than 
from  the  direct  solution  of  quartz. 

Amorphous  silica  in  the  form  of  "  flint "  plays  a 
conspicuous  part  in  the  constitution  of  many  soils  in 
the  south  and  east  of  England;  owing  to  their 
durability  and  the  former  greater  extension  of  the 
chalk,  they  are  found  in  many  districts  remote  from 
the  chalk,  even  in  the  drift  beds  of  the  Channel  Islands. 
When  first  won  from  the  chalk,  flints  possess  a  clear 
black  translucent  structure,  and  are  easily  fractured  and 
crushed ;  when  weathered,  either  in  flint  gravels  or 
on  the  surface  of  the  soil,  they  become  yellow  or  brown 
in  colour,  more  opaque,  and  much  harder,  so  that 
weathered  flints  are  always  preferred  for  road-making. 
The  surface  also  becomes  covered  with  a  white  incrusta- 
tion, extending  to  a  depth  of  Tx¥  of  an  inch  or  more ; 
this  is,  however,  only  incipient  weathering,  probably 
due  to  the  freezing  of  the  small  amount  of  water  that 
soaks  in  at  the  surface. 

The  Felspars  constitute  the  most  important  group  of 
minerals  found  in  the  crystalline  rocks :  they  are 
double  silicates  of  alumina  and  some  other  base,  potash, 
soda,  or  lime,  of  the  general  formula  R2O,  A12O3,  6SiO2, 
where  RZO  may  be  either  K2O,  Na2O,  or  CaO.  In 
granites  and  gneisses  the  common  felspar  is  orthoclase 
or  potash  felspar ;  in  the  volcanic  rocks  plagioclase 
felspars  predominate,  in  which  the  base  is  lime,  gener- 
ally with  some  admixture  of  soda  and  potash. 

The  felspars  are  all  distinguished  by  the  ease  with 
which  they  are  attacked  by  water  containing  carbonic 
acid,  those  containing  lime  more  readily  so  than  the 


I.]  •      ROCK-FORMING  MINERALS  19 

potash  felspar.  The  lime  or  the  alkali  is  removed  in 
solution,  some  of  the  silica  is  also  removed  ;  the  alumina 
remains  as  a  hydrated  silicate,  A12O3,  2SiO2,  2H2O, 
called  kaolinite.  Owing  to  this  disintegration  of 
felspar,  the  crystalline  rocks  in  which  felspar  is  present 
weather  rapidly,  the  other  materials,  quartz,  mica, 
hornblende,  become  loosened  from  the  matrix,  and 
the  whole  rock  becomes  rotten.  The  granite  of  Corn- 
wall and  Devon  is  generally  covered  to  a  considerable 
depth,  as  much  as  100  feet  in  some  cases,  with  a 
layer  of  kaolinite,  in  which  the  unchanged  quartz  and 
mica  are  embedded ;  the  kaolinite,  freed  by  washing 
from  the  quartz,  mica,  etc.,  forms  the  "  china  clay "  or 
kaolin  of  commerce.  In  the  same  way  the  basalts  and 
other  kindred  rocks  give  rise  to  a  red  clay,  consisting 
of  kaolinite  and  the  red  iron  oxides  resulting  from 
the  oxidation  of  the  magnetite  and  the  hornblende, 
augite,  etc.,  which  contain  ferrous  silicates.  From  the 
decomposition  of  the  felspars,  augite,  hornblende,  etc., 
all  our  clays  arise ;  as  these  minerals  also  generally 
contain  potash,  they  are  the  source  of  the  potash 
required  by  crops,  which  is  always  more  abundant  as 
clay  predominates  in  the  soil. 

Daubree  caused  3  kilos  of  fragments  of  felspar 
to  revolve  in  an  iron  cylinder  with  3  litres  of  water, 
so  that  they  practically  performed  a  journey  of  460 
kilometres,  with  the  result  that  2-72  kilos  of  mud  were 
formed,  of  which  36  grams  were  clay,  and  in  the 
water  there  were  12-6  grams  of  potash  in  solution  as 
silicate. 

Senft  examined  the  action  of  water  charged  with 
carbonic  acid  upon  two  granites,  one  (A)  composed  of 
orthoclase,  quartz,  and  potash  mica,  the  other  of  (B) 
plagioclase,  quartz,  and  magnesia  mica,  and  obtained 
in  solution — 


20 


THE  ORIGIN  OF  SOILS 


[CHAP. 


A. 

B. 

Potash  as  Bicarbonate 
Soda  as  Bicarbonate     . 

15  to  25  per  cent 
2   .,     6        „ 

5  to    8  per  cent. 
8  „   10        „ 

Lime  as  Bicarbonate    . 

I    „      2          „ 

4  ..     5        ). 

Magnesia  as  Bicarbonate 

a  trace 

io  „   15 

Silica  .... 

a  little 

a  little 

Iron  as  Bicarbonate     . 

a  trace 

a  trace 

The  undissolved  residue  of  A  was  a  white,  of  B  a 
yellow,  clay  containing  fragments  of  quartz  and  flakes 
of  mica. 

The  following  analyses  show  the  change  that  takes 
place  in  passing  from  orthoclase  felspar  to  kaolin ;  in 
the  third  column  the  analysis  of  kaolin  is  recalculated 
to  show  what  arises  from  100  parts  of  felspar,  on  the 
assumption  that  none  of  the  alumina  is  removed  by 
solution : — 


Orthoclase 
Felspar. 

Kaolin. 

Kaolin 
from  100  Felspar. 

Silica         .         . 
Alumina   . 
Potash 
Water       . 

64-2 
1  8-4 
17 

46-8 

37-3 
2-5 
13 

23-1 
18-4 
I«J 
6-4 

99.6 

99.6 

49 

Mica  is  essentially  a  double  silicate  of  alumina  and 
potash,  with  some  oxide  of  iron :  the  potash  being 
replaced  by  magnesia  in  black  mica  or  biotite.  Mica 
splits  up  into  minute  flakes  as  the  rock  weathers,  but 
these  flakes  are  fairly  resistent  to  chemical  change,  and 
may  be  detected  in  most  sands  and  sandstones.  Ulti- 
mately, however,  they  pass  into  hydrated  silicates  of 


I.] 


ROCK-FORMING  MINERALS 


21 


alumina,   and   are   rarely  to   be   detected    in   the   soils 
resting  upon  sedimentary  rocks. 

Hornblende  and  Augite,  though  differing  in  crystalline 
shape,  are  chemically  identical,  and  consist  of  silicates  of 
varying  proportions  of  lime,  magnesia,  alumina,  ferrous 
and  ferric  oxides ;  manganese  and  the  alkali  metals  are 
generally  also  present  They  constitute,  with  plagioclase 
felspar  and  magnetic  oxide  of  iron,  the  chief  part  of  the 
rocks  that  are  sometimes  roughly  termed  "  greenstone  " — 
basalts,  diorites,  etc.,  of  both  volcanic  and  plutonic 
origin.  They  decompose  under  the  action  of  carbonic 
acid  charged  water,  especially  those  containing  much 
lime,  while  those  with  much  magnesia  are  the  most 
resistent ;  the  products  of  the  action  are  kaolinite, 
oxides  of  iron,  and  carbonates  of  lime  and  magnesia. 
The  following  analysis  (Ebelmar)  show  the  chemical 
change  in  the  weathered  layers  of  a  basalt  from 
Bohemia  and  a  greenstone  or  dolerite  from  Cornwall : — 


BASALT. 

GREENSTONK. 

Unaltered. 

Weathered. 

Unaltered. 

Weathered. 

Silica 

44-4 

42-5 

51-4 

44-5 

Potash    . 

4.8 

1-2 

Soda 

2-7 

/        2  °      I 

3-9 

I  '7 

Lime 

11.3        |          2.5 

5-7 

1.4 

Magnesia 

9.1 

3-3 

2.8 

2-7 

Alumina. 

I2«2 

17.9 

15.8 

22-1 

Iron  Protoxide 

I2-I 

12.9 

... 

Iron  Peroxide 

3-5 

"•5 

3-0 

17.6 

Titanium  Oxide 

trace 

1-2 

0-7 

1-0 

Water     . 

4-4 

20-4 

i-7 

8-6 

The  loss  amounts  to  about  44  per  cent,  in  the 
case  of  the  basalt,  and  34  per  cent,  in  that  of  the 
greenstone. 

Another  example   may   be   given   of  the    analysis 


22 


THE  ORIGIN  OF  SOILS 


[CHAP. 


(Hanamann)  of  a  basalt  from  Bohemia,  with  that  of 
the  weathered  crust  and  of  the  resulting  soil : — 


Rock. 

Weathered  Crust. 

Soil. 

Silica 

41-84 

39'7 

39-17 

Potash     . 

0-82 

0-83 

o-94 

Soda 

3-45 

2-51 

103 

Lime 

n«l6 

8-02 

4.72 

Magnesia 

3-63 

3«20 

2-92 

Alumina  . 

i7-5i 

16-94 

16-58 

Iron  Protoxide 

3-7i 

... 

Iron  Peroxide  . 

12-77 

I5-05 

14.22 

Phosphoric  Acid 

0-5 

0-48 

0-48 

Carbonic  Acid  . 

0-88 

2-67 

0-6  1 

Water 

3-56 

10-5 

19-28 

Olivine  is  essentially  a  silicate  of  magnesia  and 
protoxide  of  iron,  not  uncommon  in  some  basalts, 
which  easily  weathers  and  becomes  a  soft  hydrated 
silicate,  called  serpentine,  to  which  talc  is  very  similar 
in  composition.  These  magnesian  silicates  are  not 
of  great  importance  in  the  British  Islands ;  only  in  the 
Lizard  district  of  Cornwall  are  they  extensively 
developed  and  give  rise  to  poor,  barren  soils. 

Calcium  Carbonate,  though  present  in  many  of  the 
older  rocks  in  its  crystalline  form  of  Calcite  or  Iceland 
Spar,  is  there  to  be  regarded  rather  as  a  secondary  product 
brought  by  infiltering  water  than  an  original  mineral. 
//  is  soluble  in  water  charged  with  carbonic  acid ; 
hence  when  the  complex  silicates  containing  lime 
are  weathered,  the  lime  is  removed  in  this  form.  The 
calcium  carbonate  is  redeposited  when  the  water  loses 
the  carbonic  acid  either  by  evaporation  or  by  diffusion 
on  contact  with  air.  In  a  massive  form  calcium  carbon- 
ate forms  many  of  the  sedimentary  formations — the 
older  ones  hardened  to  limestones,  and  the  more  recent 
ones  soft  like  the  chalk ;  in  these  cases  it  has  been  secreted 


I.]  CARBONATE  OF  LIME  23 

from  natural  waters  by  living  organisms,  foraminifera, 
corals,  etc,  and  only  gets  a  crystalline  structure  by 
later  change.  Calcium  carbonate  from  organic  sources 
is  present  to  some  extent  in  nearly  all  sedimentary 
rocks ;  the  vast  majority  of  the  fossils  there  found  are 
constituted  of  calcite. 

In  the  limestone  and  chalk  rocks  the  calcium  car- 
bonate is  never  quite  pure ;  in  the  white  chalk,  which 
is  the  purest,  the  proportion  of  calcium  carbonate,  after 
excluding  the  flints,  is  only  about  98  per  cent. ;  in 
others  the  proportion  of  clay  and  mud  which  were 
simultaneously  deposited  gradually  increases,  so  that 
we  can  find  rocks  of  every  gradation  between  chalk  and 
clay  or  sandstone. 

Owing  to  its  solubility,  the  weathering  of  limestone 
takes  the  form  of  the  removal  of  calcium  carbonate 
more  or  less  completely,  leaving  a  fine-grained  residue 
of  the  insoluble  clay  or  sand.  In  the  case  of  chalk 
and  of  the  purer  limestones,  the  insoluble  residue  con- 
sists mainly  of  a  fine  red  or  yellow  clay ;  the  chalk 
downs,  when  not  obscured  by  drift  formations,  are 
covered  with  a  sticky,  reddish  soil,  only  as  a  rule  a 
few  inches  in  thickness,  and  though  the  actual  chalk 
is  so  close,  in  many  cases  this  soil  is  almost  deprived 
of  all  its  calcium  carbonate.  Almost  exactly  similar 
material  may  be  obtained  in  the  laboratory  by  dis- 
solving a  few  pounds  of  chalk  or  limestone  in  dilute 
hydrochloric  acid.  Whenever  a  section  is  exposed  in 
chalk  or  limestone  rocks,  it  will  be  noticed  that  the 
dividing  line  between  soil  and  rock  is  very  irregular ; 
thin  as  the  soil  may  be  as  a  whole,  in  places  it  descends 
into  cavities  and  "  pipes "  in  the  rock,  sometimes  20  or 
30  feet  deep.  In  these  depressions  the  soil  is  the  same 
reddish  clay  as  occurs  on  the  surface,  mixed  with  flints 
in  the  case  of  the  upper  chalk ;  they  are  essentially  the 


24  THE  ORIGIN  OF  SOILS  [CHAP. 

results  of  solution,  and  represent  the  lines  along  which 
the  drainage  of  the  rain  water  has  been  more  active, 
owing  to  a  joint  or  fissure  in  the  rock  below. 

Other  minerals  which  do  not  constitute  any  large 
proportion  of  the  earth's  crust,  but  still  play  some 
part  in  the  soil,  are  apatite,  glauconite,  selenite,  limon- 
ite,  and  iron  pyrites. 

Apatite,  or  crystallised  phosphate  of  lime, — 
Ca5(PO4)3F, — is  present  in  small  quantities  in  many 
of  the  fundamental  rocks,  and  is  probably  the  ultimate 
source  of  the  phosphoric  acid  of  soils.  Apatite  also 
occurs  massive  in  some  of  the  older  strata,  and  has  been 
worked  as  a  raw  material,  for  the  manufacture  of  phos- 
phatic  manures,  in  Norway  and  Canada. 

Selenite,  hydrated  sulphate  of  lime,  CaSO4,  2H2O, 
termed  gypsum  when  massive,  is  not  a  fundamental 
mineral,  but  occurs  in  most  clay  rocks  in  well  de- 
veloped crystals.  Diffused  through  the  soil  and  dis- 
solved in  soil  water,  selenite  doubtless  provides  most  of 
the  sulphur  required  by  plants. 

Limonite,  hydrated  oxide  of  iron,  occurs  in  lumps 
and  bands  in  many  of  the  sedimentary  rocks ;  in  a 
diffused  state  it  is  the  main  colouring  matter  of  soils; 
in  heavy,  undrained  soils  it  often  forms  a  layer  or  "  pan  " 
some  inches  below  the  surface.  It  is  deposited  from 
water  containing  bicarbonate  of  iron  on  exposure  to  the 
air ;  the  rusty  deposits  and  stains  from  chalybeate  springs 
and  wells  consist  of  limonite.  The  action  appears  to  be 
as  follows — the  hydrated  peroxides  of  iron  in  the  soil 
when  in  contact  with  humus  (decayed  vegetable  matter) 
and  water  charged  with  carbonic  acid  become  first 
reduced  to  the  ferrous  state  by  the  organic  matter,  and 
then  dissolved  as  bicarbonate.  On  exposure  to  the 
air,  the  excess  of  carbonic  acid  escapes  by  diffusion, 
the  ferrous  carbonate,  as  it  is  precipitated,  is  also  oxi- 


I.]  ROCK-FORMING  MINERALS  25 

dised  by  the  oxygen  of  the  air,  and  deposited  as  limonite. 
It  will  be  noticed  that  stones  taken  from  peaty  land 
are  always  bleached  white,  through  the  removal  of  iron, 
and  the  surface  sand  of  heathy  land  is  always  simi- 
larly bleached.  On  examining  a  section  of  any  purely 
sandy  formation,  the  surface  soil  will  be  found  to  be 
bleached  below  the  layer  of  vegetable  matter  to  the 
depth  of  a  foot  or  more.  Then  comes  a  layer  an  inch  or 
two  thick  nearly  black  in  colour,  where  the  sand  is  more 
or  less  cemented  together  by  limonite,  and  below  this 
the  normal  brown  or  yellow  sand  begins.  The  black 
band  is  formed  at  the  depth  to  which  the  air  usually 
penetrates  the  soil ;  it  consists  of  limonite  deposited 
at  the  evaporating  surface  of  the  soil  water,  which 
contains  the  iron  dissolved  from  the  bleached  surface 
sand.  In  a  similar  manner  arises  the  hard  layer  of 
limonite,  the  "  iron  pan  "  or  "  moor-band  pan,"  found  just 
below  the  cultivated  soil  on  many  undrained  lands,  and 
again  the  deposit  of  " bog  iron  ore"  which  is  generally 
to  be  seen  beneath  the  black  peaty  accumulation  in  any 
swampy  place.  The  solution  of  iron  as  bicarbonate, 
and  its  precipitation  as  limonite,  do  not  occur  in  soils 
containing  any  calcium  carbonate,  being  essentially  a 
sign  of  an  acid  condition  of  the  soil  and  its  need  for 
lime  or  chalk. 

Glauconite  is  a  hydrated  silicate  of  iron,  alumina, 
and  potash  with  a  little  lime  and  magnesia,  which 
occurs  as  dark  green  grains  in  many  sedimentary  rocks, 
especially  of  the  Cretaceous  age.  It  is  to  the  presence  of 
this  material  that  the  Greensand  formations  owe  their 
name ;  it  is  sometimes  also  to  be  seen  in  chalk  and 
in  the  tertiary  sandstones.  It  readily  weathers  to 
brown  oxides  of  iron. 

Zeolites.  Akin  to  glauconite  are  certain  hydrated 
double  silicates  of  aluminium  and  the  alkalis  or  alkaline 


26  THE  ORIGIN  OF  SOILS  [CHAP. 

earth,  called  generically  zeolites,  which  play  a  very 
important  part  in  the  soil,  though  they  may  not  be 
present  in  large  amounts.  These  bodies,  which  result 
from  the  weathering  of  the  felspars,  contain  a  consider- 
able proportion  of  water,  loosely  combined  and  readily 
displaced,  but  their  distinguishing  feature  and  the  ease 
with  which  the  secondary  bases  they  contain,  the  calcium, 
magnesium,  sodium  or  potassium,  are  replaced  by  other 
metals,  whenever  their  salts  are  brought  into  contact 
with  the  zeolites.  Little  is  known  of  the  actual  nature  of 
the  zeolitic  bodies  in  the  soil,  but  certain  zeolites  occur 
from  time  to  time  in  a  pure  state.  The  best  known  of 
them  is  natrolite,  which  crystallises  in  fine  needles 
possessing  the  composition — Na2O,  A12O3,  3SiO2)  2H2O, 
a  little  calcium  being  generally  present  also. 

Iron  Pyrites,  FeS2,  occurs  in  small  brass  yellow 
cubic  crystals  in  many  of  the  older  rocks,  especially 
those  of  a  clay  character;  another  form,  in  fibrous 
masses  of  a  lighter  colour,  is  called  marcasite,  and  is 
common  in  the  more  modern  clays,  especially  the 
London  clay,  and  again  in  round  balls  in  the  chalk. 
Marcasite  readily  oxidises  in  moist  air  to  ferrous  sul- 
phate and  sulphuric  acid :  and  many  clay  soils  contain 
basic  sulphates,  soluble  in  dilute  acids  but  not  in  water, 
that  have  arisen  in  this  way.  Selenite  and  the  soluble 
sulphates  present  in  well  waters,  especially  in  clay  soils, 
are  probably  secondary  products  arising  from  the  oxida- 
tion of  marcasite.  In  a  finely  divided  condition  iron 
pyrites  forms  the  colouring  matter  of  many  dark  green 
or  olive  rocks  and  clays. 

Soil  and  Subsoil. 

Although  the  transition  from  soil  to  subsoil  is 
gradual,  the  distinction  between  the  two  is,  as  a  rule, 
easy  to  be  made ;  the  change  begins  an  inch  or  so 


I.]  SOIL  AND  SUBSOIL  27 

below  the  usual  limit  of  cultivation  on  arable  soils,  on 
pastures  at  the  depth  to  which  the  mass  of  the  roots 
penetrate.  The  most  obvious  difference  between  the 
two  lies  in  the  comparative  richness  of  the  staple  in 
decaying  vegetable  matter  or  humus,  which  indeed 
would  be  entirely  confined  to  the  surface  layers  were 
it  not  for  the  decay  of  the  deeper  roots  and  the  work  of 
worms.  To  the  humus  is  also  due  the  difference  in 
colour ;  not  only  does  the  colour  deepen  towards  black 
as  the  proportion  of  humus  increases,  but  by  it  the 
sands  and  clay  are  to  a  greater  or  less  extent  bleached 
through  the  removal  of  the  iron  oxides  which  colour 
them,  hence  the  inorganic  material  is  lighter  and  duller 
in  colour  in  the  soil  than  in  the  subsoil.  In  stiff  clays 
the  subsoil  often  shows  signs  of  imperfect  oxidation  at 
comparatively  slight  depths.  On  an  old  pasture  on  the 
Gault  Clay  a  trench  was  dug,  the  top  3  inches  were  black 
or  nearly  so  and  gradually  changed  to  a  stiff  brown 
loam  which  extended  to  a  depth  of  9  or  10  inches, 
becoming  lighter  and  more  distinctively  yellow  as 
the  admixture  of  humus  diminished  ;  below  this  depth 
the  clay  became  mottled,  grey,  and  yellow  mixed,  till 
at  a  depth  of  4  feet  practically  the  whole  was  a  dark 
blue  unweathered  clay,  owing  its  colour  to  iron  pyrites 
and  glauconite  or  kindred  silicates  of  iron  protoxide. 
One  of  the  greatest  distinctions  between  soil  and  subsoil 
lies  in  their  respective  texture ;  in  humid  climates  like 
our  own  the  soil  is  almost  invariably  composed  of  coarser 
grains  than  the  subsoil,  though  in  arid  climates  soil 
and  subsoil  appear  to  be  almost  uniform.  This  is  due 
to  the  rain  constantly  percolating  through  even  the 
stiffest  soils  and  washing  down  the  finest  particles  ;  in 
heavy  rains  also,  water  runs  off  the  surface  into  the 
ditches,  carrying  with  it  the  finest  particles  of  the  soil 
and  leaving  behind  the  coarser  grains  on  the  surface. 


28  THE  ORIGIN  OF  SOILS  [CHAP. 

Naturally,  this  loss  of  the  finer  particles  is  greater  as 
the  soil  is  more  worked  and  made  open  to  percolation 
and  washing ;  to  some  extent  it  is  counterbalanced  by 
the  work  of  worms  bringing  the  fine  mould  to  the 
surface  from  below,  so  that  the  difference  is  least  in 
an  old  pasture.  Per  contra,  it  is  greatest  in  an  old 
garden  soil,  where  the  constant  working  and  further 
opening  of  the  soil  by  the  introduction  of  bulky  manure 
often  results  in  so  complete  a  washing  down  of  all  the 
finer  particles  that  the  soil  proper  loses  its  power  of 
cohering,  falls  into  dust  when  dry,  and  is  popularly  said 
to  be  "  worn  out." 

In  addition  to  its  humus  the  soil  is  nearly  always 
richer  than  the  subsoil  in  all  the  essential  elements  of 
plant  food,  despite  the  fact  that  crops  have  been  raised 
on  it  for  generations ;  the  crops,  in  fact,  have  been  the 
cause  of  the  difference,  for  the  deeper  roots  draw  food 
from  the  subsoil  and  leave  it  behind  on  the  surface  as 
the  plants  decay.  Potash  is  perhaps  an  exception  in 
this  connection ;  being  essentially  a  product  of  the 
weathering  of  felspar,  and  removable  from  the  soil  by 
water  containing  carbonic  acid,  it  is  often  more  abundant 
in  the  comparatively  unweathered  subsoil.  The  richness 
of  the  humus,  its  greater  warmth  and  the  freer  access 
of  air  also  cause  it  to  be  more  abundantly  supplied  with 
those  organisms  which  play  such  an  important  part  in 
preparing  the  food  of  the  higher  plants :  as  will  be 
seen  later,  subsoils  become  almost  without  living 
organisms  at  a  very  slight  depth. 

For  all  these  reasons, — the  absence  of  humus,  and  of 
the  organisms  associated  with  it,  the  comparative  poverty 
in  inorganic  plant  food,  the  presence  sometimes  of  unoxi- 
dised  material,  and  on  stiff  soils  the  great  change  of 
texture, — the  subsoil  is  often  comparatively  unfertile 
and  may  be  almost  barren.  Desirable  as  it  is  to  work 


I.]  CLASSIFICATION  OF  SOILS  29 

the  subsoil  and  open  it  to  the  access  of  air  and  the  free 
penetration  of  roots,  all  methods  of  cultivation  should 
be  avoided  that  would  bury  the  surface  soil  and  bring 
the  subsoil  to  the  top.  A  plough  which  inverts  the 
soil  should  not  go  below  the  former  limit  of  cultivation, 
and  if  it  is  desired  to  deepen  this  limit,  it  should  be 
done  by  degrees,  half  an  inch  or  so  each  year.  Immense 
damage  has  been  done  to  the  fertility  of  many  of  the 
heavier  soils  by  rash  ploughing  with  steam,  especially 
where  the  old  "  lands  "  were  thrown  down,  burying  the 
fertile  soil  in  the  furrows  and  baring  the  raw  clay  on 
the  tops  of  the  ridges. 

General  Classification  of  Soils. 

Although  a  distinction  has  been  drawn  between 
sedentary  soils  and  soils  of  transport,  there  are  few 
sedentary  soils  that  do  not  contain  material  which  has 
been  carried  from  some  other  formation  at  a  distance ; 
only  on  great  stretches  of  flat  country  belonging  to  a 
single  geological  formation  may  be  expected  a  soil 
purely  derived  from  the  rock  below.  Especially  in 
Britain,  where  the  outcrops  of  the  different  formations 
are  generally  narrow,  and  where  the  surface  is  always 
undulating,  we  find  that  the  continual  creeping  of  soil 
particles  to  lower  levels  has  resulted  in  an  admixture 
of  foreign  material  in  most  soils.  "  La  couche  tres- 
mince  de  la  terre  veg^tale  est  un  monument  d'une 
haute  antiquite "  (Elie  de  Beaumont),  so  that  in  many 
places  the  soil  contains  the  debris  of  formations  now 
removed  by  denudation.  In  the  south-east  of  England 
the  soils  that  rest  on  the  chalk,  which  may  be  only 
from  a  few  inches  to  a  few  feet  below,  contain 
abundance  of  quartz  sand,  even  up  to  75  per  cent 
No  such  sand  exists  in  the  chalk  itself,  so  that  it  has 
come  from  the  lower  tertiary  beds  which  once  over- 


30  THE  ORIGIN  OF  SOILS  [CHAP. 

spread  the  chalk.  On  the  wide  flats  of  Weald  Clay 
in  the  same  district,  the  soil  contains  sand  that  has  crept 
from  the  central  hills  of  the  Weald  or  from  the  Lower 
Greensand  escarpment,  often  several  miles  away.  The 
following  analysis  of  a  soil  resting  on  a  brick  earth 
bed  in  the  valley  of  the  Kentish  Stour,  shows  that  the 
brick  earth,  which  itself  contains  little  or  no  chalk,  has 
become  covered  with  chalky  rain-wash  from  the  hills 
flanking  the  valley  : — 

Depth— Inches   .      .      o  to  6        6  to  12         12  to  18         18  to  24 
Calcium  Carbonate  %        9-20  7-16  2-6  0-96 

In  the  main,  however,  the  bed  below  gives  its  char- 
acter, both  chemical  and  physical,  to  the  soil ;  and  the 
ordinary  rough  classification  of  soils  into  sands,  clays, 
marls,  and  loams,  follows  closely  the  nature  of  the 
underlying  geological  stratum.  A  coarse  -  grained 
sandstone  gives  rise  to  a  typically  sandy  soil,  such  as 
the  soils  derived  from  the  Bagshot  beds,  which  form 
the  New  Forest  and  the  heathy  land  in  the  Aldershot 
district ;  on  the  Lower  Greensand  lie  the  sandy  heaths 
in  west  Surrey,  Hampshire,  and  in  Beds  ;  again,  on 
the  Bunter  beds. of  the  New  Red  Sandstone  lie  many 
of  the  uncultivated  commons  and  parks  of  the  Midlands, 
such  as  Sutton  Park,  Cannock  Chase,  and  Delamere 
Forest  These  coarse  sandy  soils,  which  have  so  often 
remained  unenclosed  as  forests  and  commons,  are  gener- 
ally deficient  in  chalk,  and  accumulate  peat  wherever  a 
parting  of  clay  gives  rise  to  stagnant  water. 

Clay  soils  are  common  in  nearly  every  part  of 
Britain ;  they  arise  from  the  great  clay  strata  of  all 
ages,  like  the  London  Clay,  the  Weald  Clay,  and  the 
Oxford  Clay,  or  from  metamorphic  rocks  like  slate,  or 
from  the  crystalline  rocks  like  granite  and  basalt,  or 
even  from  the  limestones  by  solution. 


I.]  CLASSIFICATION  OF  SOILS  31 

Between  the  sands  and  the  clays  come  mixtures  of 
all  grades,  better  working  than  the  clays  and  more 
fertile  than  the  pure  sands ;  sometimes  the  clay  forma- 
tion itself  contains  sand,  as  in  the  upper  beds  of  the 
London  Clay,  or  we  may  have  a  fine-grained  sandstone 
mixed  with  clay,  as  in  some  of  the  carboniferous  rocks. 
In  all  these  cases,  when  chalk  is  absent,  and  drainage 
incomplete,  there  will  be  an  accumulation  of  humus, 
resulting  in  a  peaty  formation. 

Some  argillaceous  limestones  give  rise  to  typical 
"  marls,"  mixtures  of  chalk  and  clay ;  e.g.,  some  of 
the  beds  of  the  Lias  and  of  the  Keuper. 

Other  limestones  with  a  sandy  basis,  and  fine- 
grained sandstones  cemented  by  carbonate  of  lime, 
give  rise  to  "  loams,"  which  are  free-working  soils,  mainly 
composed  of  fine  sand  with  some  clay  and  a  little 
calcium  carbonate.  The  alluvial  soils  in  the  valleys 
are  loams,  passing  in  places  into  gravels ;  these  are 
generally  the  richest  soils  ;  as  a  rule  they  are  mixtures 
derived  from  many  formations,  and  so  are  well  supplied 
with  humus  and  the  mineral  elements  of  plant  food  ; 
they  are  deep,  and  not  over  consolidated,  thus  admitting 
of  the  percolation  of  water  and  the  descent  of  roots  ;  yet 
they  are  fine-grained  enough  to  prevent  them  drying 
out  too  rapidly.  But  though  these  terms,  sands,  clays, 
marls,  loams,  and  peaty  soils,  serve  for  rough  descriptive 
purposes,  a  more  exact  determination  of  the  constituent 
particles  is  necessary  to  properly  characterise  a  soil, 
and  for  this  we  must  resort  to  what  is  termed  the 
"  mechanical  analysis  "  of  a  soil. 


CHAPTER  II 

THE  MECHANICAL  ANALYSIS  OF  SOILS 

Nature  of  Soil  Constituents :  Sand,  Clay,  Chalk,  and  Humus — 
Methods  of  Sampling  Soils — Methods  for  the  Mechanical 
Analysis  of  a  Soil — Interpretation  of  Results. 

IT  has  already  been  indicated  that  as  soils  are  derived 
from  the  waste  of  rocks,  they  consist  of  a  mass  of 
particles  of  various  minerals  and  of  all  sizes,  together 
with  a  certain  amount  of  humus  of  vegetable  origin, 
and  that  they  may  be  roughly  classified  according  to 
the  predominance  of  the  coarse-grained  particles  called 
" sand"  or  the  very  fine  material  known  as  " clay" 

The  mechanical  analysis  of  a  soil  consists  in  pushing 
this  rough  "  eye  and  hand  "  classification  a  stage  further 
into  the  region  of  exact  measurement,  and  in  deter- 
mining the  minute  physical  structure  of  the  soil  by 
estimating  the  proportions  in  which  particles  of  various 
sizes  are  mixed  together  in  the  soil.  Upon  the  physical 
structure  of  the  soil  so  determined,  or  as  we  should 
practically  term  it,  the  texture,  depend  some  of  its 
most  important  features,  particularly  its  behaviour  with 
regard  to  the  supply  of  water  to  crops  and  its  amena- 
bility to  cultivation. 

In  the  first  place,  it  will  be  necessary  to  discuss  a 
little  more  thoroughly  the  nature  of  the  four  substances 

82 


CHAP.  II.] 


THE  NATURE  OF  SAND 


33 


to  which  the  texture  of  the  soil  has  been  referred — the 
sand,  clay,  chalk,  and  humus — of  which  the  first  two 
are  of  most  importance,  since  soils  which  are  mainly 
characterised  by  chalk  or  humus  are  less  commonly 
in  cultivation. 

Sand. — On  the  seashore,  in  beds  of  an  alluvial 
nature,  and  in  formations  of  all  geological  ages,  we  are 
familiar  with  sand  ;  in  the  main  it  consists  of  grains  of 
quartz,  rounded  by  continual  rubbing,  and  more  or  less 
coloured  by  oxide  of  iron.  It  represents  the  quartz 
contained  in  the  fundamental  rocks,  weathered  and 
worn  by  water:  in  some  cases  of  comparatively 
recent  origin,  in  others  it  is  material  that  has  re- 
peatedly been  formed  into  a  sedimentary  rock, 
disintegrated  afresh  and  sorted  by  the  action  of  running 
water.  The  coarser  the  grains  of  which  a  sand  is  made 
up,  the  more  rapid  must  have  been  the  current  from 
which  it  was  deposited.  The  following  table  shows 
the  rate  of  flow  which  is  necessary  to  carry  sand 
grains  of  various  sizes : — 


Diameter  of  Grains, 

Velocity  of  Current, 

mm. 

mm.  per  sec. 

0-5 

64 

0-3 
0-16 

32 
16 

0-12 

8 

0-072 

4 

0047 

0036 

2 
I 

0025 

0-5 

A  closer  examination  of  most  sands  will  show  that 
they  do  not  consist  wholly  of  quartz  grains,  but  also 
contain  rounded  fragments  of  many  of  the  minerals 
present  in  the  fundamental  rocks  which  have  any 
resistance  to  weathering.  Flakes  of  mica  are  common, 

C 


34      THE  MECHANICAL  ANALYSIS  OF  SOILS   [CHAP. 

fragments  of  more  or  less  altered  felspar,  of  oxide  of 
iron,  and  even  of  tinstone,  rutile,  and  zircon,  may  be 
identified.  In  fine-grained  sands  the  fragments  of 
minerals  other  than  quartz  become  as  a  rule  more 
abundant,  till  they  begin  to  predominate  over  the 
quartz  grains  in  the  finest  silts  and  muds  that  are 
deposited  from  very  gently  moving  water.  Under  the 
microscope  the  quartz  grains  show  a  crystalline  struc- 
ture, and  a  surface  more  or  less  dulled  and  rounded 
according  to  the  travel  the  grains  have  suffered.  In 
mass  the  chief  characteristic  of  sand  is  its  want  of 
coherence  when  dry. 

Clay. — The  material  we  call  clay  is  characterised  by 
certain  properties  that  are  shown  when  the  clay  has 
been  "  puddled,"  i.e.,  kneaded  when  in  a  moist  condition. 
The  clay  \splastic,  it  can  be  moulded  and  worked  into 
various  shapes,  even  into  quite  thin  leaves,  and  it  will 
retain  these  shapes  on  drying.  During  the  drying 
process  a  shrinkage  takes  place :  the  dry  material  is 
hard  and  tenacious,  and  can  only  be  broken  or  crumbled 
with  difficulty.  The  shrinkage  is  considerable :  a  little 
brick  was  made  of  good  modelling  clay  7  inches  long, 
and  about  I  square  inch  in  section ;  two  marks  were 
then  made  on  this  6  inches  apart ;  after  a  fortnight's 
drying  in  a  room  the  marks  were  only  5-7  inches  apart, 
showing  a  shrinkage  of  5  per  cent  Clay  is  further 
impermeable  to  water  when  in  the  moist  puddled 
condition,  for  which  reason  it  is  used  to  line  the  bottoms 
of  ponds  in  pervious  soil,  and  is  built  up  inside  the 
retaining  dams  of  reservoirs ;  quite  a  thin  layer  of  clay 
will  hold  water  indefinitely  as  long  as  it  is  not  allowed 
to  dry  and  crack,  nor  to  be  washed  away  by  the  action 
of  running  water. 

From  a  chemical  point  of  view,  all  clays  are  found 
to  consist  largely  of  kaolinite,  the  hydrated  silicate 


II.]  THE  NATURE  OF  CLAY  35 

of  alumina  which  is  formed  by  the  weathering  of  fel- 
spar ;  the  other  materials  present  consist  of  extremely 
fine  grains  of  quartz  and  other  weathered  minerals, 
together  with  more  or  less  oxide  of  iron.  "  China 
clay "  and  the  best  "  pipe  clays "  contain  little  or  no 
iron ;  the  deep-seated  clay  formations  are  generally 
coloured  dark  green  or  blue  or  black  by  the  presence 
of  ferrous  silicates  like  glauconite ;  on  weathering  and 
exposure  at  the  surface  the  clays  become  yellow  or 
brown,  owing  to  the  oxidation  of  these  ferrous  to  ferric 
salts. 

Water  in  which  a  little  clay  has  been  rubbed  up 
remains  turbid  for  a  very  long  time ;  days  and  even 
weeks  elapse  before  the  particles  settle  down  to  the 
bottom — indeed,  however  long  the  liquid  may  be  at 
rest,  a  slight  haze  or  cloudiness  may  be  observed 
within  it  Schloesing  has  drawn  a  distinction  between 
the  part  of  the  clay,  amounting  to  I  or  2  per  cent 
only  of  the  whole,  which  persists  in  remaining  sus- 
pended and  the  portion  which  settles  down ;  he  has 
called  it  "colloid  clay,"  and  attributes  many  of  the 
typical  clay  properties  to  the  jelly-like  medium  of 
colloidal  matter  by  which  the  other  defined  particles 
of  the  clay  are  surrounded.  Schloesing  associates  this 
colloid  clay  with  such  typical  colloids  as  the  highly 
hydrated  forms  of  silica  and  organic  bodies  like  starch 
and  gum  which,  though  they  appear  to  be  truly  dissolved, 
yet  cannot  diffuse  through  a  membrane,  and  form,  on 
drying,  hard  non-crystalline  masses,  with  much  shrinkage 
and  a  characteristic  fracture.  But  later  researches  on 
colloids  show  that  they  are  not  essentially  different 
from  suspended  matter;  they  consist  of  particles  too 
fine  to  settle  down  in  water,  or  to  be  arrested  by  a 
filter  even  of  porous  porcelain,  but  which  are  still 
sufficiently  coarse  to  show  their  presence  when  a  strong 


36      THE  MECHANICAL  ANAL  YSIS  OF  SOILS   [CHAP. 

beam  of  light  is  passed  through  the  liquid,  as  is  not  the 
case  with  bodies  truly  dissolved.  From  this  point  of  view 
the  "colloid  clay"  would  only  represent  the  limiting 
state  of  fineness,  differing  in  degree,  but  not  in  kind, 
from  the  other  clay  particles. 

The  question  still  remains  whether  we  shall  give 
to  clay  a  physical  or  a  chemical  definition ;  in  the  first 
place,  does  the  fineness  of  the  material  alone  confer  the 
characteristic  clay  properties  of  plasticity,  impermeability 
to  water,  and  shrinkage  and  tenacity  on  drying,  or  do 
these  properties  depend  on  the  chemical  composition  of 
the  substance  making  up  the  clay.  It  is  easy  to  show 
that  fineness  of  division  is  a  necessary  factor  in  the 
existence  of  clay,  because  we  can  obtain  material 
possessing  the  chemical  composition  of  typical  clays 
which  yet  behave  physically  as  if  they  were  sand.  A 
sample  of  crude  kaolinite  rock  as  dug  in  Cornwall 
from  the  surface  of  granite,  was  powdered  and  passed 
through  a  sieve  retaining  all  particles  above  0-2  mm. 
in  diameter;  the  remainder,  which  consisted  mainly 
of  kaolinite  with  a  little  mica,  was  further  separated 
by  sedimentation  from  water  into  four  fractions  : — 


Fraction. 

Approximate  Size  of 
Particles  in  mm. 

Per  cent,  of 
Original  Material. 

I 

02       tO      OO5 

22 

2 

005  „    o-oi 

39 

3 
4 

o-oi    „     0-005 
below  0-005 

21 
20 

Of  these  fractions  the  first  contained  all  the  mica, 
the  others  were  practically  pure  kaolinite,  yet  the  second 
fraction  showed  none,  and  the  third  very  little  of  the 
characteristic  properties  of  clay ;  when  dried  they  fell 


II.] 


THE  NATURE  OF  CLA  Y 


37 


or  could  easily  be  rubbed  into  a  fine  powder,  only 
the  fourth  and  finest  fraction  dried  into  a  hard  co- 
herent mass.  Thus  we  can  have  material  which  con- 
sists entirely  of  kaolinite,  and  yet  is  not  clay ;  such  as 
we  see  in  natural  deposits  of  fuller's  earth,  which 
consists  of  kaolinite  but  possesses  no  plasticity,  and 
falls  on  drying  into  a  fine  powder. 

In  the  same  way  a  natural  soil  contains  particles 
of  silicates  of  alumina  of  all  sizes,  though  they  only 
begin  to  predominate  in  the  fractions  of  finest  grain. 

On  separating  one  of  the  Rothamsted  soils  into 
fractions,  according  to  their  size  by  the  method  to  be 
described  later,  and  analysing  them,  the  following  results 
were  obtained : — 


Fraction. 

Approximate  Size 
of 
Particles  in  mm. 

Per  cent, 
of 
Original 
Soil. 

Percentages  in  Material. 

Silica. 

Ferric 
Oxide. 

Alumina. 

I 

O-2      to   004 

24 

94-6 

I-I 

3'4 

2 

0-04    „     O-OI 

35 

92-0 

1-2 

6-2 

3 

o-oi    „     0-004 

II 

88-3 

1-8 

8-5 

4 

O-004  »      O-OO2 

6 

61.7 

70 

23-4 

5 

below  0-002 

24 

45-9 

12-2 

30-9 

Taking  the  mean  of  several  analyses,  the  fifth  fraction, 
which  is  to  be  regarded  as  clay  proper,  possessed  the 
following  approximate  composition,  if  all  the  alumina  is 
combined  as  A12O3,  2SiO2,  2H2O — kaolinite  72  to  75 
per  cent ;  ferric  oxide,  1 1  to  12  per  cent. ;  quartz,  9 
to  10  per  cent ;  alkalis  and  alkaline  earths,  4  to  6  per 
cent 

From  these  results  we  must  conclude  that  kaolinite 
is  not  necessarily  clay,  but  that  fineness  of  grain  is  also 
an  essential  factor,  the  characteristic  clay  properties  not 
being  developed  except  in  material,  the  particles  of 


38      THE  MECHANICAL  ANAL  YSIS  OF  SOILS   [CHAP. 

which  are  less  than  one-fivehundredth  of  a  millimetre  in 
diameter. 

But  though  fineness  of  grain  is  a  factor,  it  is  probably 
not  the  only  factor,  as  may  be  seen  from  a  consideration 
of  another  important  property  of  clay — its  power  of 
flocculating  or  coagulating  under  the  action  of  minute 
quantities  of  various  salts.  To  illustrate  this  point,  a  few 
grams  of  good  clay  should  be  rubbed  up  with  several 
litres  of  distilled  water,  and  the  supernatant  turbid  liquid 
poured  off  into  a  series  of  tall  jars  each  holding  from 
300  to  500  cc.  of  the  liquid.  To  one  of  these  jars 
nothing  is  added,  to  two  others  -018  and  0-009  gram  of 
hydrochloric  acid  respectively,  to  a  fourth  0-028  gram  of 
calcium  chloride,  and  to  the  fifth  0-58  gram  of  sodium 
chloride.  The  contents  of  the  jars  are  shaken  up  until 
solution  is  effected,  and  they  are  then  put  aside  to  stand. 
After  some  time  the  liquids  to  which  the  salts  have  been 
added  will  begin  to  clear,  and  the  clay  particles  will  clot 
together  and  fall  to  the  bottom ;  the  jar  containing  the 
larger  quantity  of  hydrochloric  acid  will  clear  the  first, 
the  others  will  clear  approximately  together,  but  the  pure 
clay  water  will  remain  turbid  for  many  days.  If  a  little 
of  the  turbid  clay  water  be  examined  by  a  ^inch 
oil  immersion  lens  under  the  microscope,  it  is  just 
possible  to  see  the  clay  particles  in  rapid  "  Brownian  " 
motion,  and  if  a  little  acid  or  salt  be  then  introduced 
under  the  cover  glass,  they  will  be  seen  to  move  together 
and  form  into  little  clots  or  aggregates  as  soon  as  they 
experience  the  effect  of  the  added  acid  or  salt.  By 
comparative  experiments  it  can  be  shown  that  the 
flocculating  power  of  any  salt  is  proportional  to  its  amount 
up  to  a  certain  limit,  when  the  material  is  so  completely 
flocculated  that  no  further  addition  of  salt  has  any 
effect ;  conversely,  the  flocculating  power  of  a  given 
amount  of  salt  is  inversely  proportional  to  the  quantity 


30 

HNO,  .        . 

28 

H2SO4  . 

20 

15 

Ca(N03)2     . 

10 

CaSO4  . 

•    >5 

3 

KN03  .        . 

>2 

K^CV 

•    <i 

>i 

KC1      . 

<i 

Na,jSO4 

.     0-5 

ii.]  FLOCCULAT10N  39 

of  clay  suspended  in  the  liquid.  The  flocculating  power 
of  a  salt  also  varies  with  both  the  acid  and  the  metal ; 
the  following  table  shows  approximately  their  com- 
parative effect : — 

HC1      . 

CaCl2     . 
KC1 

NaCl      . 

The  alkalis  and  salts  like  phosphate  of  sodium,  which 
give  rise  to  free  alkalis  on  hydrolysis,  instead  of 
flocculating  have  the  opposite  effect,  and  keep  the 
particles  in  their  finest  state  of  division  without  any 
tendency  to  settle. 

It  is,  furthermore,  possible  to  show  that  many 
substances,  however  finely  divided,  will  not  assume  the 
condition  of  indefinite  suspension  in  water  so  as  to  be 
flocculated  by  salts ;  in  particular,  suspensions  of  finely 
divided  quartz,  ferric  hydrate,  and  hydrated  alumina 
flocculate  spontaneously  and  will  not  remain  turbid  for 
many  minutes,  though  in  their  turn  they  can  be 
deflocculated  and  made  to  remain  in  suspension  by 
adding  a  trace  of  free  alkali  to  the  liquid. 

Without  going  further  into  the  details  of  a  subject 
which  is  still  very  obscure,  the  condition  of  free  suspen- 
sion in  water  and  the  Brownian  motion  of  the 
particles  of  clay  seem  to  be  associated  with  the  presence 
of  the  zeolitic  double  silicates  which  contain  atoms  of 
potassium  or  sodium  in  their  molecule,  and  which  doubt- 
less give  rise  to  a  little  free  alkali  by  their  partial 
hydrolysis  when  in  contact  with  a  large  bulk  of  water. 

We  may  thus  conclude  that  fineness  of  grain  is 
not  the  only  factor  in  the  constitution  of  clay,  but  that 
the  characteristic  clay  properties  which  are  always 
associated  with  the  power  of  flocculation  depend  also 


40      THE  MECHANICAL  ANALYSIS  OF  SOILS   [CHAP. 

upon  the  nature  of  the  material ;  in  the  soil  they  depend 
upon  the  presence  of  the  zeolitic  double  silicates  derived 
from  the  weathering  of  the  felspars  in  the  fundamental 
rocks. 

The  power  of  flocculation  plays  a  very  important 
part  in  the  cultivation  of  clay  soils.  When  such  a  soil 
possesses  a  good  texture  its  finest  particles  are  in  a 
state  of  temporary  aggregation  or  flocculation,  so  that 
they  behave  as  if  the  soil,  as  a  whole,  were  built  up  of 
much  coarser  particles.  Just  as  a  potter  or  a  brick- 
maker  brings  his  material  into  its  highest  condition  of 
plasticity  by  repeatedly  kneading  and  working  it,  by 
which  process  the  naturally  formed  aggregates  are 
resolved  into  their  ultimate  particles  and  the  material  is 
made  as  fine-grained  as  possible,  so  if  a  clay  soil  be  in 
any  way  worked  or  disturbed  when  in  a  wet  condition,  it 
becomes  apparently  more  clayey  than  before.  It  remains 
persistently  wet  and  impervious  to  the  percolation  of 
water,  and  shrinks  when  dry  into  hard  tenacious  clods. 
But  if  the  clay  be  exposed  to  the  weather  for  some  time, 
so  that  it  undergoes  alternations  of  temperature,  freez- 
ing and  thawing,  wetting  and  drying,  it  will  experience 
a  certain  amount  of  spontaneous  flocculation  and  behave 
as  though  it  were  coarser  grained,  so  that  if  caught 
in  the  right  state  of  partial  dryness  it  may  easily  be 
crumbled. 

Flocculation  may  also  be  aided  or  otherwise  by  the 
use  of  certain  artificial  manures,  as  will  be  explained 
later ;  the  incorporation  again  of  humus  much  improves 
the  texture,  while  the  action  of  lime  is  particularly 
effective  and  is  much  employed  in  practice  to  ameliorate 
the  working  of  clay  soils. 

Lime  itself  can  be  shown  in  the  laboratory  to  possess 
little  flocculating  power,  for  though  its  base  is  calcium,  a 
highly  effective  metal,  it  is  combined  as  a  hydrate,  which 


II.]  CALCIUM  CARBONATE  41 

has  a  deflocculating  effect  However,  as  soon  as  lime 
is  applied  to  the  soil  it  becomes  converted  into  carbonate, 
and  some  of  it  will  be  always  going  into  solution  as 
bicarbonate,  a  salt  which  possesses  great  flocculating 
power. 

In  practice,  the  application  of  such  small  quantities 
of  lime  as  a  ton  or  even  half  a  ton  to  the  acre  have  the 
greatest  value  in  ameliorating  the  working  of  clay  land  ; 
not  only  does  it  move  more  readily  and  fall  more  easily 
into  a  good  tilth,  but  by  becoming  coarser  grained  it 
allows  the  rain  to  percolate  more  freely  and  thus  dries 
earlier  in  the  season,  so  that  the  limed  land  can  often  be 
worked  several  days  before  the  unlimed  land  can  be 
touched.  Though  the  Rothamsted  soil  is  by  no  means 
of  the  heaviest,  it  is  only  because  of  the  repeated 
additions  of  carbonate  of  lime  in  former  years  that 
it  can  be  retained  under  arable  cultivation ;  portions  of 
the  same  land  without  carbonate  of  lime  lie  so  wet 
in  the  spring  that  they  were  laid  down  to  grass  in 
consequence  of  the  repeated  failures  to  secure  a  good 
seed  bed. 

Chalk,  or  carbonate  of  lime,  is  present  in  all  soils, 
with  the  exception  of  a  few  extremely  open  sands  and 
peaty  soils  that  are  practically  of  vegetable  origin. 
The  proportion  varies  enormously,  according  to  the 
origin  of  the  soil ;  on  some  of  the  thin  loams  derived 
directly  from  the  great  calcareous  formations  like  the 
chalk  or  the  oolite,  the  calcium  carbonate  in  the  soil 
may  rise  to  as  high  a  proportion  as  60  per  cent,  but  in 
the  majority  of  the  loams  under  cultivation  the  pro- 
portion is  nearer  i  per  cent,  and  it  often  falls  much 
below  this  in  clays  and  sands.  Chalk  in  the  soil  is 
essentially  a  transitory  substance,  as  it  is  constantly 
removed  by  the  action  of  percolating  water  charged 
with  carbonic  acid,  arising  from  the  decay  of  vegetable 


43      THE  MECHANICAL  ANAL  YSIS  OF  SOILS   [CHAP. 

matter  in  the  surface  soil  Many  of  the  fermentation 
changes  that  also  take  place  in  this  vegetable  matter 
give  rise  to  acids,  which  in  their  turn  combine  with 
the  calcium  carbonate.  So  rapid  are  these  removals  of 
calcium  carbonate  that  it  is  difficult  to  understand  how 
any  of  it  persists  in  the  surface  layers  of  many  soils,  the 
subsoil  of  which  shows  that  they  must  have  been  initially 
poor  in  chalk,  were  there  not  some  compensating 
agencies  at  work.  Amongst  these  agencies  must  be 
reckoned  the  calcium  salts  in  plants,  which  in  many 
cases  are  drawn  up  by  deep-seated  roots  from 
the  subsoil  and  become  calcium  carbonate  on  the 
ultimate  decay  of  the  plant  tissues. 

In  a  normal  soil  the  particles  of  calcium  carbonate 
are  of  all  sizes,  many  of  the  finer  particles  of  silt  and 
clay  are  loosely  cemented  together  by  calcium  carbonate, 
as  may  be  seen  by  the  increase  in  the  finer  fractions 
if  a  soil  be  washed  with  dilute  acid  before  it  is  separ- 
ated by  sedimentation. 

Humus. — On  examining  many  rocks  taken  from 
such  depths  that  they  have  undergone  none  of  the 
weathering  processes  which  convert  them  into  soil,  they 
are  found  to  contain  both  carbon  and  nitrogen,  occasion- 
ally in  quantities  comparable  with  those  found  in  the 
soil  itself.  This  is  only  the  case  with  the  sedimentary 
rocks  and  particularly  the  indurated  clays,  the  carbon 
and  nitrogen  in  fact  only  represent  the  organic 
matter  in  the  original  deposit  in  a  more  or  less 
mineralised  condition.  But  since  these  carbon  and 
nitrogen  compounds  are  only  slightly  affected  by  any  of 
the  weathering  processes  by  which  soil  is  made,  they 
must  pass  into  the  soil  and  there  become  merged  with 
the  organic  matter  of  more  recent  origin.  Such  material, 
however,  plays  a  very  unimportant  part  in  the  soil,  and 
we  may  pass  on  at  once  to  the  debris  of  vegetation  of 


II.]  THE  NATURE  OF  HUMUS  43 

recent  origin  or  the  humus  which  is  characteristic  of  all 
soils  proper. 

The  term  humus  is  applied  to  the  black  or  dark 
brown  material  of  vegetable  origin  which  gives  to 
surface  soil  its  characteristic  darker  colour  as  compared 
with  the  subsoil.  It  is  essentially  a  product  of  bacterial 
action ;  there  are  a  number  of  bacteria  working  in  the 
absence  of  air  and  universally  distributed,  which  attack 
the  carbon  compounds  of  plant  tissues,  especially  the 
carbohydrates,  with  the  production  of  marsh  gas  or 
hydrogen,  carbonic  acid,  and  humus.  In  the  presence 
of  air  the  characteristic  humus-forming  fermentation 
is  replaced  by  one  which  results  in  the  complete  com- 
bustion of  the  organic  matter  to  carbonic  acid.  For 
this  reason  more  humus  is  found  in  a  pasture  than  in  a 
continually  aerated  arable  soil,  more  again  in  clays  than 
in  the  lighter  soils  through  which  air  is  always  being 
drawn  as  the  rain  percolates,  and  the  accumulation 
of  humus  reaches  its  maximum  where  considerable 
rainfall  and  an  impermeable  stratum  combine  to  make 
the  soil  so  water-logged  that  all  access  of  air  is  cut  off, 
as  in  swamps  and  bogs.  The  presence  of  chalk  in  the 
soil  also  assists  in  the  destruction  of  humus,  since  it 
neutralises  the  acids  which  largely  compose  the  humus, 
and  which  tend  to  inhibit  the  further  action  of  bacteria. 

The  chemical  composition  of  humus  is  indefinite ; 
it  is  a  variable  mixture  of  several  substances,  themselves 
of  very  complex  constitution  ;  it  always  contains  more 
carbon  and  less  hydrogen  and  oxygen  than  the  vege- 
table tissues  from  which  it  was  formed.  The  following 
figures  show  the  composition  of  grass  and  of  the  top 
brown  layer  of  turf  in  a  peat  bog,  also  of  the  same  peat 
of  greater  age  at  depths  of  7  and  14  feet,  the  mineral 
matter  and  moisture  being  excluded  in  calculation  in 
each  case : — 


44      THE  MECHANICAL  ANALYSIS  OF  SOILS   [CHAP. 


Grass. 

Top  Turf. 

Peat  at  7'. 

Peat  at  14'. 

Carbon    . 

50-3 

57-8 

62 

64 

Hydrogen 

5-5 

5-4 

5-2 

5 

Oxygen   . 

42-3 

36 

30-7 

26-8 

Nitrogen 

1-8 

0-8 

2*1 

4.1 

Substances  akin  to  humus  can  be  formed  from  the 
carbohydrates  (such  as  sugar,  starch,  and  cellulose),  by 
heating  them  for  some  time  with  water  under  pressure, 
the  action  being  more  rapid  if  a  trace  of  mineral  acid  be 
present ;  the  resulting  substances  are  weak  acids  and 
form  salts,  so  are  generally  termed  humic  acid : — 


HUMIC  ACID. 

From  Sugar. 

Natural. 

Carbon 
Hydrogen 
Oxygen     . 
Nitrogen   . 

63-9 
4.6 

31-5 

56-3    to    59 
4-4     „       4-9 
32-7     „     36 
2-8     „       3-6 

As  a  rule,  the  active  humus  of  the  soil  is 
there  present  in  the  form  of  salts  of  calcium,  which 
on  treatment  of  the  soil  with  dilute  hydrochloric  acid 
are  decomposed,  a  little  of  the  humic  acids  going  into 
solution  but  the  greater  part  remaining  undissolved. 
By  filtering  off  the  acid  and  then  treating  the  soil  with 
a  weak  (4  per  cent,  by  volume)  solution  of  ammonia  or 
other  alkali,  the  liberated  humic  acids  are  dissolved  and 
may  be  reprecipitated  either  as  free  acids  by  the  addition 
of  hydrochloric  acid,  or  as  calcium  salts  by  the  addition 
of  a  solution  of  calcium  chloride.  The  humic  acids  thus 
going  into  solution  are  sometimes  estimated  as  "  soluble 
humus,"  they  do  not  include  the  whole  of  either  the 
organic  matter  or  the  nitrogen  in  the  soil.  The  brown 


II.]  THE  NATURE  OF  HUMUS  45 

solution  that  is  formed  is  akin  to  the  dark  liquid  draining 
from  a  dung  heap,  which  contains  humus  dissolved 
by  the  alkaline  carbonates  of  the  fermented  urine. 

Occasionally  soils  are  found  which  naturally  pos- 
sess an  acid  reaction,  and  in  which  the  whole  or  part 
of  the  soluble  humus  is  uncombined  with  calcium, 
so  that  it  goes  into  solution  in  ammonia  without  the 
preliminary  treatment  with  acid.  The  portion  of  the 
natural  humus  of  soils  that  is  soluble  in  acids  contains 
nitrogen,  and  seems  to  be  of  the  nature  of  an  amide. 

Although  dark  brown  humic  substances  can  be 
prepared  from  carbohydrates,  and  therefore  contain 
only  carbon,  hydrogen,  and  oxygen,  yet  the  soluble 
humus  of  the  soil,  even  when  dissolved  and  reprecipi- 
tated,  always  contains  some  nitrogen,  nor  can  it  be 
obtained  entirely  free  from  phosphorus  and  mineral 
matter.  The  original  vegetable  matter  is  made  up  not 
only  of  carbohydrates,  but  of  other  carbon  compounds 
containing  nitrogen,  and  in  some  cases  both  nitrogen 
and  phosphorus ;  these  all  break  down  under  bacterial 
action  into  dark-coloured  substances  richer  in  carbon, 
and  roughly  classed  as  humus.  The  splitting-up  process 
continues  in  the  soil,  so  that  humus  becomes  one  of  the 
great  sources  of  nitrogen  for  the  food  of  plants,  and  a 
soil  well  supplied  with  humus  is  generally  regarded  as 
fertile. 

During  the  formation  and  continued  decomposition 
of  humus  the  carbohydrates  appear  to  be  first  attacked, 
and  the  nitrogen-containing  bodies,  e.g.,  the  nucleins  in 
particular,  resist  the  action  of  bacteria.  For  this  reason, 
where  we  find  the  proportion  of  humus  in  a  soil  is  low, 
the  proportion  of  nitrogen  in  the  humus  itself  will  be 
high,  the  decay  of  the  humus  falls  more  heavily  on  the 
purely  carbonaceous  part  of  the  material. 

This  is  seen  in  the  figures  obtained  by  Lawes  and 


46      THE  MECHANICAL  ANAL  YSIS  OF  SOILS   [CHAP. 

Gilbert  for  the  ratio  that   exists  between   the  propor- 
tions of  carbon  and  nitrogen  in  various  soils  : — 


RATIO  -^ 

N 

Cereal  Roots  and  Stubble 
Leguminous  Stubble 
Dung 

Very  old  Grass  Land     . 
Manitoba  Prairie  Soils  . 
Pasture  recently  laid  down 
Arable  Soil 
Clay  Subsoil 


43 
23 
18 

13-7 

13 

n-7 


Hilgard  and  Jaffa  also  found  that  the  humus  of  soils 
in  an  arid  climate,  where  the  deficiency  of  rainfall  causes 
the  soil  to  be  very  open,  contains  a  higher  proportion  of 
nitrogen  than  is  found  in  the  humus  of  damper  soils : — 


Number 
of  Samples 

Kxaimnrd. 

Average  per  cent. 
of 
Humus  in  Soil. 

Average  per  cent. 
of 
Nitrogen  in  Humus. 

Arid  Soils 
Semi-arid  Soils 
Moist  Soils    . 

18 
8 
8 

o-75 
0.99 

3-04 

I5-87 
10-03 

5-24 

The  following  table  (p.  47)  gives  the  results  of  the 
determination  of  carbon,  nitrogen,  humus,  and  the 
percentage  of  nitrogen  in  the  humus,  in  a  selection  of 
extremely  rich  virgin  soils  obtained  from  different 
parts  of  the  world  ;  the  Canadian,  Russian,  and  Monte 
Video  soils  were  very  similar  uniform  fine-grained  grey 
or  black  soils  found  on  the  great  plains. 

These  results  would  seem  to  indicate  that  the  most 
valuable  humus,  i.e.  that  which  will  decay  rapidly 
and  yield  nitrogen  compounds  available  as  food  for 
plants,  is  that  possessing  a  high  ratio  of  carbon  to 
nitrogen. 


II.] 


THE  NATURE  OF  HUMUS 


47 


3 

S3 

a 

g 

& 

0 

Kg 

Locality. 

Description  of 
Boll. 

| 

I 

o 

w 

1 

EH 
ws 

23 

to 

* 

a 
1 

-5 

I.  Canada 

Indian  Head 

Black  Prairie 

2-59 

0-317 

8-2 

3-94 

4-07 

2.  Canada 

Wide  Awake 

() 

2-58 

o-33o 

7-8 

2-71 

3.  Russia  . 
4.  Rhodesia 

Ploty 
Salisbury 

Black  Steppe 
Black  Vlei 

2-19 
20-15 

0-268 
1-89 

8-2 

10-7 

5-66 
21.3 

2.42 
2-40 

Rlarlf            1 

5.  Monte  Video 

| 

liiilt  K.                 1 

"  Camp  "  Soil/ 

1-89 

0-261 

7-3 

4.48 

3-51 

6.  New  Zealand 

/*    Tararua 
\   Mountains 

Black  Sandy 
Pasture 

12-66 

0-949 

13-2 

10-35 

4-67 

Against  this,  Berthelot  and  Andr£  have  investigated 
the  ratio  of  carbon  to  nitrogen  in  the  different  portions 
of  the  humus  which  can  be  dissolved  by  alkalis  or  acids, 
and  they  find  that  the  most  soluble  portions  contain  the 
highest  proportion  of  nitrogen.  It  does  not,  however, 
follow  that  the  substances  most  soluble  in  acids  or 
alkalis  are  necessarily  those  which  will  most  readily  be 
converted  by  bacteria  into  a  form  available  for  plants, 
and,  on  the  whole,  the  evidence  seems  to  show  that  a 
humus  rich  in  nitrogen  will  yield  it  very  slowly  to  crops. 

Humus  acts  as  a  weak  cement  and  holds  together 
the  particles  of  soil,  thus  it  serves  both  to  bind  a  coarse- 
grained sandy  soil,  and,  by  forming  aggregates  of  the 
finest  particles,  to  render  the  texture  of  a  clay  soil  more 
open.  In  determining  the  sizes  of  the  constituent 
particles  of  a  soil,  the  "  mechanical  analysis,"  it  is  desir- 
able to  remove  the  humus  as  far  as  possible,  and  so 
break  up  these  temporary  aggregates. 

Sampling  of  Soils. 

The  first  step  in  the  analysis  of  any  soil,  mechanical 
or  chemical,  consists  in  obtaining  a  sample  that  shall 
adequately  represent  the  land  in  question. 


48      THE  MECHANICAL  ANAL  YSIS  OF  SOILS   [CHAP. 

In  this  country  it  is  customary  to  take  a  sample  down 
to  a  depth  of  9  inches  as  representing  the  soil  proper ; 
it  is,  however,  doubtful  if  this  is  not  too  deep,  being 
below  the  depth  to  which  cultivation  is  generally  carried  ; 
probably  a  6-inch  sample  would  more  truly  represent  the 
cultivated  soil.  In  many  cases  it  will  be  found  that  the 
true  soil  does  not  extend  to  a  depth  of  anything  like  9 
inches,  but  that  there  is  a  sharp  change  into  subsoil  or 
even  rock  before  this  point :  e.g.,  on  the  chalk  downs  the 
soil  is  often  not  more  than  4  inches  deep,  below  which 
white  broken  chalk  rock  begins.  In  such  cases  the 
sample  must  only  be  taken  to  the  depth  at  which  the 
visible  change  begins. 

To  obtain  the  sample  two  methods  are  generally 
adopted.  At  Rothamsted  a  steel  box,  without  top  or 
bottom,  9  inches  deep,  and  6  inches  square  in  section,  is 
used ;  the  sides  are  wedge-shaped,  about  f  inch  thick 
at  the  top  and  tapering  off  to  cutting  edges  below. 
The  surface,  if  uneven  arable  land,  is  first  raked  over 
and  gently  beaten  level,  then  the  box  is  placed  in 
position  and  driven  down  with  a  heavy  wooden  rammer 
till  the  top  of  the  box  is  flush  with  the  surrounding 
soil.  The  soil  enclosed  by  the  box  is  then  carefully  dug 
and  scraped  out  into  a  bag  for  conveyance  to  the  labora- 
tory ;  two  or  three  samples  to  the  same  depth  being 
taken  from  the  same  field  and  afterwards  mixed. 
Should  samples  of  the  subsoil  be  required,  the  box 
is  left  in  position  after  its  contents  have  been  scraped 
out,  and  the  surrounding  soil  is  dug  away  to  the  9-inch 
level,  the  box  is  then  rammed  down  for  the  second 
9  inches,  and  its  contents  removed :  the  process  being 
repeated  till  the  required  depth  has  been  reached. 

A  modification  of  the  Rothamsted  method  consists  in 
marking  out  on  the  surface  a  square  9  inches  on  the 
side,  and  digging  away  the  surrounding  soil  until  a 


FlG.  I. — Photograph  of  Soil-sampling  Tools. 


[To  face  page  49. 


n.]  METHODS  OF  SAMPLING  49 

9-inch  cube  of  earth  remains  standing ;  over  this  a 
wooden  box  is  slipped,  and  the  cube  is  cut  off  by  pushing 
a  spade  beneath  at  the  9-inch  level. 

On  soils  which  do  not  contain  many  large  stones, 
samples  may  be  taken  with  an  auger,  both  more  rapidly 
and  with  greater  security  of  obtaining  an  average 
sample.  A  convenient  tool  for  the  purpose  consists  of  a 
cylindrical  auger  made  of  steel,  about  TV  inch  thick,  of  2 
inches  internal  diameter  and  12  inches  deep,  with  a  slot 
f  inch  wide  running  from  top  to  bottom  ;  the  lower 
edge  of  the  cylinder  and  the  edges  of  the  slot  are 
sharpened ;  to  the  upper  end  of  the  cylinder  a  handle 
carrying  a  wooden  crossbar  is  riveted.  The  auger  is 
forced  gently  into  the  soil  with  a  twisting  motion  until 
the  required  depth  is  reached,  when  the  tool  is  with- 
drawn and  the  core  scraped  out  into  a  bag.  Six  to  ten 
cores  at  least  are  taken  at  regular  intervals  in  the  same 
field  and  mixed  to  secure  an  average  sample.  Each 
boring  can  be  continued  to  obtain  subsoil  samples  as 
deep  as  the  length  of  the  handle  permits.  It  is  impos- 
sible to  obtain  samples  with  the  auger  when  the  soil  is 
dry.  Fig.  i  shows  a  photograph  of  both  types  of  soil- 
sampling  tools. 

When  the  samples  reach  the  laboratory  they  are 
spread  out  on  shallow  trays  to  dry,  which  process  may 
be  accelerated  by  a  gentle  warmth,  not  exceeding  40°  C. 
In  dealing  with  stiff  soils  it  is  advisable  to  crumble  all 
the  lumps  by  hand  while  the  earth  is  still  somewhat 
moist.  When  the  whole  is  sensibly  dry  the  stones  are 
separated  by  a  sieve  having  round  holes  3  mm.  in 
diameter ;  the  material  that  does  not  pass  the  sieve 
is  gently  worked  up  in  a  mortar  with  a  wooden  pestle, 
care  being  taken  not  to  break  the  stones,  chalk,  etc, 
but  only  to  crush  the  lumps  of  earth.  Finally,  the 
material  upon  the  sieve  is  roughly  weighed  and  well 

D 


jo      THE  MECHANICAL  ANALYSIS  OF  SOILS   [CHAP. 

washed  in  a  stream  of  water  till  all  the  fine  earth  is 
gone,  dried,  picked  over  to  free  it  from  roots  and 
stubble,  and  weighed  as  "stones."  To  get  the  pro- 
portion borne  by  the  stones  to  the  soil,  the  fine  earth 
is  also  weighed,  an  addition  being  made  of  the  weight 
lost  by  the  stones  in  washing. 

Of  course  the  figure  obtained  for  the  proportion  of 
stones  is  only  approximate,  for  if  the  stones  are  of  any 
size  they  will  be  very  irregularly  caught  by  the  auger 
or  even  by  the  6-inch  square  tool.  The  material  passing 
the  sieve  is  again  spread  out  in  a  thin  layer  in  an 
ordinary  room,  until  the  surface  maintains  the  same 
colour  as  the  lower  layers ;  it  is  then  bottled  up  as 
"  air-dry  fine  earth  "  for  analysis. 

The  Mechanical  Analysis  of  a  Soil. 

The  mechanical  analysis  that  follows  consists  in 
dividing  the  fine  earth  into  a  series  of  fractions  con- 
sisting of  particles  of  known  size ;  we  can  use  sieves  to 
sort  out  the  coarser  grades,  but  the  finer  ones  must  be 
separated  by  their  relative  powers  of  remaining  sus- 
pended in  water. 

The  methods  in  use  depend  on  two  principles : 
in  one,  the  hydraulic  method  (Hilgard,  Schoene,  Nobel), 
soil  is  washed  by  successive  currents  of  water  of  veloci- 
ties calculated  to  carry  particles  of  the  required  size 
according  to  the  table  on  p.  33 :  in  the  other,  the  sedi- 
mentation method  of  Osborne,  Knop,  and  Schloesing, 
the  soil  is  suspended  in  water  and  allowed  to  stand,  the 
separation  being  effected  either  by  the  times  required 
for  the  particles  to  settle  down  through  a  fixed  distance, 
or  by  the  distances  fallen  in  a  given  time.  The  method 
to  be  described  is  based  upon  the  latter  principle.  The 
hydraulic  method  requires  special  apparatus,  and  is  only 
suited  to  laboratories  entirely  devoted  to  soil  analysis. 


II.]  ANALYTICAL  METHODS  51 

Method  of  Analysis. 

1.  Ten  grams  of  the  air-dry  fine  earth  are  weighed 
out  into  a  beaker  or  basin  and  treated  with  100  c.c.  of 
TV/5  hydrochloric  acid  ;  the  soil  is  well  worked  up  with  a 
rubber  pestle  (made  by  fixing  a  glass  rod  into  a  small 
solid  rubber  bung)  until  all  the  lumps  of  clay,  etc.,  are 
broken  up.     If  the  soil  contains  much  calcium  carbonate, 
a  further  addition  of  acid  may  be  required. 

The  object  of  the  acid  is  to  dissolve  the  carbonates  and 
humates,  and  thus  loosen  the  particles  in  any  aggre- 
gates where  chalk  or  humus  form  the  cement.  With- 
out this  preliminary  treatment  the  amount  of  clay 
found  will  be  largely  determined  by  the  proportion 
of  humus  present  ;  the  soil  of  an  arable  field,  for 
example,  will  show  more  clay  than  the  soil  of  an 
adjoining  pasture,  when  the  sedimentation  is  made  with 
water  alone.  But  after  the  preliminary  treatment  with 
acid  to  remove  the  humus,  both  fields  will  show  the 
same  proportion  of  clay  (as  they  should  do,  since  they 
are  of  the  same  origin),  and  only  differ  in  the  amount 
of  humus  they  have  accumulated — a  temporary  factor. 

After  standing  with  the  acid  for  an  hour,  the  whole 
is  thrown  on  a  tared  filter  and  well  washed  until  all  acid 
is  removed.  The  filter  and  its  contents  are  dried  and 
weighed ;  the  loss  the  soil  has  suffered  represents  the 
material  dissolved  and  the  hygroscopic  moisture. 

2.  The    soil    is    now    washed    off   the    filter    with 
ammoniacal    water    (about    i    c.c.   of   strong   ammonia 
solution  in  half  a  litre  of  water)  on  to  a  small  sieve  of 
100  meshes  to  the  linear  inch,  the  portion  passing  through 
being  collected  in  a  beaker  which  is  marked  on  the  side 
at  a  distance  of  8-5  cm.  from  the  bottom. 

The  ammonia  completes  the  dissolution  of  the  humates, 
and  also  masks  the  effect  of  any  traces  of  soluble  salts 
which  may  be  left  and  would  cause  aggregation  in  the 
manner  indicated  earlier,  p.  38. 


52      THE  MECHANICAL  ANAL  YS1S  OF  SOILS   [CHAP. 

The  portion  which  remains  on  the  sieve  is  dried  and 
weighed.  It  is  then  divided  into  "  fine  gravel "  and 
"  coarse  sand  "  by  means  of  a  sieve  with  round  holes  of 
I  mm.  in  diameter,  the  portion  retained  by  the  sieve 
being  designated  "  fine  gravel." 

3.  The  portion  in  the  beaker  is  well  worked  up  with 
the  rubber  pestle,  ammoniacal  water  is  added  up  to  the 
8-5  cm.  mark,  and  the  whole  is  put  aside  to  stand  for 
twenty-four  hours.     The  turbid,  supernatant   liquid   is 
then  rapidly  poured  off  into  a  large  jar,  and  the  residue 
is  rubbed  up  again  with  the   rubber  pestle   and   more 
ammoniacal  water,  as  before.     The  whole  operation  of 
filling  to  the  mark,  standing  for  twenty-four  hours,  and 
pouring  off  the   turbid   liquid    is    carried   through    as 
before,  and   repeated   as   long   as  any  matter  remains 
in  suspension   for  twenty-four  hours.     Generally  seven 
to  ten  decantations  will  be   sufficient,  after  which  the 
united  turbid  liquid  is  evaporated  to  dryness  in  a  tared 
basin,  and  weighed.     This  fraction  consists  of  the  "clay" 
particles  less  than  0-002  mm.  in  diameter,  together  with 
all  the  soluble  and  some  of  the  insoluble  humus.     The 
contents  of  the  dish  are  ignited  over  an  Argand  burner 
for  some  time  and  reweighed,  to  obtain  the  weight  of  the 
"  clay  "  after  ignition. 

4.  The   sediment   from   which    the    clay   has    been 
removed  is  worked  up  as  before  in  the  beaker,  which, 
however,   is  now   only   filled   to   the  depth  of  7-5  cm. 
The  contents    are    now    allowed   to  stand   for  twelve 
and   a   half  minutes   only,   when   the  liquid  is  poured 
off   into    a    large    jar     as     before.      The     operations 
are     then    repeated    until     all     the     sediment    settles 
in   twelve   and   a  half  minutes   and   the   liquid  above 
is  left  quite  clear.     The  contents  of  the  jar  are  now 
evaporated  to  dryness   and   weighed,   as   in   operation 
3,  before   and   after   ignition ;    this   fraction    is    desig- 


ANALYTICAL  METHODS 


53 


nated   "  fine  silt,"  and  lies  between  o-oio  and  0-002  mm. 
in  diameter. 

5.  The  sediment  remaining  in  the  beaker  is  worked 
up  afresh  just  as  in  the  previous  operations,  the  mark 
being  now  placed  10  cm.  from  the  bottom  of  the  beaker, 
and  the  time  of  settlement  fixed  at  one  hundred  seconds. 
The  sediment  is  dried  and  weighed  as  "  fine  sand,"  while 
the  portion  that  is  poured  off  is  obtained  by  evaporation 
as  in  the  previous  operations,  and  is  designated  as  "  silt." 
The  soil  has  thus  been  divided  into  the  following  series 
of  fractions : — 


Diameter  in  Millimetres. 

Maximum. 

Minimum. 

I 

Stones  and  Gravel 

3 

(Separated 

2 

Fine  Gravel 

3 

I 

by 

3 

Coarse  Sand 

I 

O-2 

sifting. 

4 

5 

Fine  Sand 
Silt  . 

O-2 
004 

0-04 
O-OI 

I    Separated 

6 

7 

Fine  Silt  . 
Clay 

O-OI 
O-OO2 

0-OO2 

1         *? 
1  subsidence. 

If  there  be  much  "  fine  gravel "  in  the  soil,  it  is  best  to 
make  a  separate  determination  of  its  amount  on  a 
sample  weighing  50  grams,  treating  with  acid  as  before, 
and  then  washing  the  whole  on  to  the  i  mm.  sieve. 
The  result  obtained  should  be  taken  as  the  true 
percentage,  and  the  other  percentages  found  in  the 
analysis  of  10  grams  only  should  be  recalculated  to  agree 
with  it 

The  sizes  of  the  particles,  the  depth  of  the  liquid,  and  the 
times  adopted  above,  are  purely  conventional.  The  time 
of  settlement  required  to  obtain  a  fraction  of  any 
given  range  of  size  can  be  determined  by  a  series  of 
trials,  the  material  remaining  suspended  in  each  case 


54      THE  MECHANICAL  ANALYSIS  OF  SOILS  [CHAP. 

is  measured  under  the  microscope  until  the  right  time 
is  hit  off  to  secure  the  desired  range  of  size  in  the 
sediment.  The  relationship  between  the  time  of  settle- 
ment, the  height  of  the  liquid  column,  and  the  diameter 
of  the  particles,  is  governed  by  the  formula  :  — 


where  a  is  the  density  of  the  particle,  a  its  radius,  p  the 
density,  and  17  the  coefficient  of  viscosity  of  the  liquid. 
The  application  of  the  formula,  however,  requires  to  be 
checked  by  observation  with  the  microscope,  because 
the  particles  are  not  spheres. 

The  hygroscopic  moisture  and  the  loss  on  ignition 
also  require  determination,  which  is  described  under 
the  chemical  analysis  of  a  soil. 

Interpretation  of  Results. 

It  is  as  yet  impossible  to  predict  the  behaviour  of 
a  soil  under  cultivation  from  a  consideration  of  its 
mechanical  analysis  ;  in  a  general  way  we  can  see 
whether  a  soil  is  heavy,  whether  it  is  likely  to  dry 
"steely,"  or  whether  it  will  crumble  readily  under 
proper  cultivation,  and  whether  it  is  more  suitable  for 
market  gardening  or  wheat  growing,  but  the  more 
refined  points  of  difference  connected  with  the  manage- 
ment of  given  soils,  which  become  known  by  experience 
to  a  good  practical  farmer,  cannot  as  yet  be  deduced 
from  the  analysis.  It  is  necessary  to  accumulate  more 
data,  until  we  possess  the  mechanical  analysis  of  a 
large  number  of  soils  whose  texture  and  amenability 
to  cultivation  have  been  ascertained  by  long  practice  ; 
then  we  shall  be  able  to  assign  any  soil  by  its  mechanical 
analysis  to  a  known  type. 

The  power  of  a  soil  to  retain  moisture  and  resist 
moderate  drought  depends  on  a  predominance  of  the 


II.]  TYPICAL  SOILS  55 

finer  particles  and  of  humus ;  good  wheat  land  or 
land  that  will  form  sound  permanent  pasture  will 
contain  at  least  30  per  cent,  of  silt  and  clay.  The 
ease  with  which  a  soil  suffers  the  rain  to  percolate 
depends  upon  the  relatively  low  proportion  of  silt  and 
clay  rather  than  on  the  amount  of  coarse-grained 
material ;  the  fine  particles  pack  in  among  the  larger, 
and  the  soil  is  equally  resistent  to  the  passage  of  water, 
whether  the  finest  material  is  diffused  among  coarse 
sand  and  gravel,  or  among  the  finer  grades  of  sand. 
The  shrinkage  of  a  soil  on  drying,  and  its  tenacity  when 
dry,  are  even  more  dependent  on  low  proportions  of 
coarse  sand,  humus,  and  chalk,  than  on  the  actual 
amount  of  clay  and  silt  which  cause  the  shrinkage. 
The  really  difficult  soils  to  work  are  those  containing 
less  than  20  per  cent,  of  sand  above  01  mm.  in 
diameter. 

The  table  on  page  56  will  serve  to  illustrate  these 
points. 

Soil  No.  i  represents  one  of  the  lightest  of  sands, 
about  the  extreme  limit  of  cultivation — a  soil,  indeed, 
which  had  been  found  unfit  for  ordinary  farming, 
and  had  been  planted  with  conifers. 

It  will  be  seen  that  more  than  83  per  cent,  consists 
of  "  sand,"  nearly  all  of  the  coarser  kinds,  while  the  clay 
only  amounted  to  4-7  per  cent,  most  of  which  was  really 
ferric  oxide.  Calcium  carbonate  is  also  entirely  absent, 
owing  to  which  the  soil  accumulates  more  humus  than 
would  be  expected  from  its  great  aeration,  and  in  the 
hollows  where  water  lies  it  often  becomes  peaty.  Such 
soils  are  rarely  in  cultivation,  but  are  left  as  wastes, 
carrying  a  natural  vegetation  of  heather  and  pine. 

Because,  however,  of  their  lightness  and  warmth, 
they  are  sometimes  valuable  for  market  gardening  on  a 
small  scale,  if  they  are  so  situated  that  large  supplies  of 


56      THE  MECHANICAL  ANAL  YSIS  OF  SOILS   [CHAP. 

farmyard  manure  or  town   dung  are   available,  straw- 
berries being  a  favourite  crop. 

Soil  No.  2  was  taken  from  the  stackyard  field  of  the 
farm  of  the  Royal  Agricultural  Society  at  Woburn,  and 
represents  a  light  sandy  loam,  early,  and  extremely  easy 


1 

2 

3 

4 

5 

6 

7 

8 

•d 

a 

S£ 

a 

I 

| 

0 

i 

g. 

l«   •" 

gg 

O  P 

S-o" 

•S, 

.a 

|>, 

$ 

5 

|2 

w  f- 

a 

.EP 

to 

a 

£ 

£ 

n 

dj 

'3 

S 

m 

Fine  Gravel 

4.1 

1-0 

3-o 

1-2 

1.9 

1-9 

1-3 

0-4 

Coarse  Sand 

70-3 

49-9 

33-8 

5-3 

3-3 

6-2 

21-2 

0-8 

Fine  Sand 

7-0 

16-1 

28-0 

32-1 

36-8 

21-4 

12-5 

6.4 

Silt 

1-5 

ii-i 

5-6 

33-3 

2I-O 

32-5 

ISO 

18-6 

Fine  Silt 

5-8 

5-6 

10-8 

5-3 

I4'3 

13-8 

11-9 

13-6 

Clay 

4-7 

9-7 

6-6 

1  1-8 

I3'5 

17.6 

28.3 

42-2 

Moisture 

2-6 

1-2 

4-3 

1.9 

1-4 

2-2 

1.6 

9-5 

Loss  on  ignition 
Calcium  Carbonate 

3;° 

3-8 

6.9 

O.2 

4-5 

O-I 

4-5 
0-3 

5-8 
2-5 

7-8 

9.1 
0-4 

SUBSOILS. 

Fine  Gravel 

6-5 

I-O 

4-1 

o-3 

2-6 

1.7 

0.7 

0-2 

Coarse  Sand 

75-5 

50-1 

36-8 

2-1 

2-8 

4-3 

1  1-6 

0.5 

Fine  Sand 

4-9 

15-9 

26-1 

270 

35-2 

15-8 

7-3 

6-2 

Silt 

1-7 

12-5 

5-4 

40-8 

19.9 

24.0 

9-8 

15-9 

Fine  Silt 

4-2 

5-9 

8-4 

5-7 

1  6.1 

16-7 

15-2 

10-2 

Clay 

2-2 

8-6 

9-5 

16.4 

16-2 

28.7 

42-7 

48-9 

Moisture 

1-6 

0-9 

3-3 

3-6 

1-2 

3-8 

2-6 

6-3 

Loss  on  ignition 
Calcium  Carbonate  . 

2-7 

2-7 

5-7 

O-I 

2-8 
O-I 

4-1 
o-3 

4.6 

O-I 

8-1 

7-3 

O-I 

to  work  in  any  weather.  Owing  to  the  preponderance 
of  coarse  sand,  it  suffers  somewhat  from  drought  and 
rarely  carries  heavy  crops  ;  and  though  responding  well 
to  manuring,  the  soil  is  hungry  and  does  not  long  retain 
organic  manures.  The  soil  contains  enough  silt  to 
possess  a  distinct  power  of  lifting  the  subsoil  water  by 


II.]  TYPICAL  SOILS  57 

capillarity,  and  similar  soils  containing  less  coarse  sand 
and  rather  more  fine  sand  and  silt  are  often  among  the 
most  valuable,  because  they  combine  free  working 
with  a  capacity  to  resist  drought  through  capillary  action. 
This  soil  is  more  suited  to  market  gardening  than  to 
mixed  farming,  makes  poor  pastures,  grows  good  barley 
and  turnips,  but  is  too  light  for  wheat  and  mangolds. 

Soil  No.  3  is  a  light  sandy  loam  from  one  of  the 
most  valued  of  the  "  red  land  "  potato  soils,  near  Dunbar. 
In  the  cool  climate,  with  a  fair  rainfall  here  prevailing, 
this  forms  an  excellent  arable  soil  for  all  crops,  specially 
prized  as  yielding  potatoes  which  retain  their  colour 
and  are  mealy  after  boiling. 

Soil  No.  4  is  a  typical  free  working  loam  from  the 
Thanet  sand  formation,  but  rather  lighter  than  usual. 
It  is  easy  to  work,  warm  and  early,  stands  drought  well, 
and  is  grateful  and  fairly  retentive  of  manure.  This  is 
a  highly  valued  soil  for  all  ordinary  arable  cultivation, 
but  is  rather  too  light  for  wheat  and  pasture  in  the 
south  or  east  of  England.  No  particular  fraction  of 
the  soil  is  predominant,  but  the  soil  is  a  fairly  uniform 
mixture  of  particles  of  all  grades. 

It  should  be  noticed  that  in  these  first  four  soils  of 
a  sandy  type  soil  and  subsoil  are  of  very  similar 
structure,  whereas  as  soon  as  the  smaller  particles 
predominate  on  the  heavy  lands,  then  the  soil  is  coarser 
grained  than  the  subsoil. 

Soil  No.  5  comes  from  the  Hastings  Sand  in  Sussex, 
and  represents  a  light  example  of  a  type  of  soil  which, 
with  a  certain  amount  of  variation  in  the  relative 
proportions  of  fine  sand  and  silt,  covers  a  considerable 
area  in  the  high  Weald  country. 

Generally  it  forms  a  sticky,  heavy  working  soil, 
commonly  described  as  a  clay,  though  the  sand  and  silt 
fractions  predominate  and  no  excessive  proportion  of 


58      THE  MECHANICAL  ANAL  YS1S  OF  SOILS  [CHAP. 

clay  is  present  The  soil,  however,  is  kept  very  close  by 
the  lack  of  coarse  sand  and  of  any  of  the  still  coarser 
gravel  and  stones,  the  absence  of  carbonate  of  lime  also 
makes  it  stickier  and  more  difficult  to  work.  If  a  good 
tilth  is  obtained,  as  for  instance  a  seed  bed  for  roots,  and 
heavy  rain  follows,  these  soils  are  particularly  liable  to 
run  together  and  set  on  drying  to  a  glazed  caked  surface, 
very  inimical  to  germination.  When  well  supplied  with 
lime  and  organic  matter,  these  soils  are  fertile  and  carry 
magnificent  crops  ;  but  they  are  rather  late  and  expensive 
to  work,  so  that  they  have  in  great  measure  been  laid 
down  to  grass.  They  carry  good  grass  when  well 
treated,  and  particularly  when  dressed  with  lime  and 
basic  slag. 

Soil  No.  6  is  taken  from  the  Broadbalk  Wheat  Field 
at  Rothamsted :  it  is  a  heavy  loam,  stubborn  and 
intractable  to  work,  which  would  lie  very  wet  were  not 
the  land  naturally  under-drained  by  the  chalk  rock  at  a 
depth  of  ten  or  twelve  feet  below.  The  surface  soil  also 
contains  a  large  number  of  flint  stones,  not  shown  in 
the  analysis,  and  these  help  to  keep  the  soil  more  open 
and  assist  the  drainage.  Heavy  as  it  is,  the  soil  is  not 
a  true  clay ;  it  is  the  silt  and  fine  sand  fractions  which 
predominate,  and  to  these  must  be  attributed  the 
tendency  of  the  soil  to  run  and  dry  with  a  caked  surface, 
if  much  rain  falls  after  a  fine  tilth  has  been  attained. 
In  the  soil  but  not  the  subsoil  there  is  a  fair  proportion 
of  calcium  carbonate,  of  artificial  origin,  and  this  con- 
tributes greatly  to  the  workability  of  the  soil,  for  it  has 
been  found  unprofitable  to  retain  some  of  the  fields, 
in  which  the  calcium  carbonate  is  absent,  under 
arable  cultivation.  Land  of  this  class  is  still  largely 
under  the  plough,  and  is  good  wheat,  mangold, 
and  bean  land,  but  is  too  heavy  for  barley  or  turnips. 
An  occasional  bare  fallow  is  desirable  to  clean  the  land 


II.]  TYPICAL  SOILS  59 

and  bring  it  into  tilth  again  ;  it  also  yields  very  fair 
permanent  pasture. 

Soil  No.  7  is  situated  on  the  Kimeridge  Clay  forma- 
tion in  Cambridgeshire ;  it  is  heavy  land,  difficult  to 
cultivate,  and  when  under  the  plough  requires  a  bare 
fallow  from  time  to  time  to  restore  the  tilth.  This 
represents  one  of  the  heaviest  soils  which  respond  to 
arable  cultivation,  which  indeed  is  only  practicable 
because  the  soil,  though  containing  so  high  a  proportion 
of  clay,  also  contains  a  good  deal  of  coarse  sand,  which 
keeps  it  open  and  helps  to  render  it  friable. 

Soil  No.  8  is  a  heavy,  undrained  London  Clay,  which 
will  carry  nothing  but  poor  pasture.  At  one  time  it 
would  carry  in  favourable  seasons  heavy  crops  of 
wheat  and  beans,  but  the  expense  of  cultivation  and 
the  danger  of  missing  a  season  have  rendered  it  quite 
unprofitable  to  farm  under  the  plough.  It  will  be 
noticed  that  the  soil  consists  almost  wholly  of  the 
finer  fractions,  nearly  one-half  being  "  clay " ;  nor 
is  there  any  difference  between  soil  and  subsoil, 
except  in  the  humus,  which  improves  the  texture 
of  the  surface. 


CHAPTER    III 

THE  TEXTURE  OF  THE  SOIL 

Meaning  of  Texture  and  Conditions  by  which  it  is  affected — Pore 
Space  and  Density  of  Soils — Capacity  of  the  Soil  for  Water — 
Surface  Tension  and  Capillarity — Percolation  and  Drainage — 
Hygroscopic  Moisture. 

IN  the  preceding  chapter,  the  nature  of  the  particles 
composing  the  soil  has  been  discussed ;  it  now  remains 
to  consider  the  manner  in  which  they  may  be  arranged, 
and  the  structure  that  results  from  the  interaction 
of  the  soil  particles,  the  water,  and  such  salts  as  may 
be  dissolved  in  the  water.  On  these  factors  depend 
what  the  farmer  knows  as  the  "texture"  of  the  soil, 
the  degree  of  resistance  it  affords  to  the  passage  of  a 
plough,  etc.,  the  ease  or  otherwise  with  which  that  prime 
object  of  cultivation,  the  preparation  of  a  seed  bed,  can 
be  attained. 

It  is  clear  that  as  a  soil  consists  of  particles  there 
must  be  between  them  a  certain  amount  of  space 
which  is  occupied  by  air  or  water ;  this  is  known  as  the 
"  pore  space,"  and  on  its  amount  will  largely  depend  the 
density  of  the  soil.  Taking  the  simplest  theoretical 
case,  a  soil  made  up  of  equal  spheres  in  contact  with  one 
another,  it  will  be  found  that  the  pore  space  is  de- 
pendent upon  the  method  of  packing,  but  not  upon  the 

60 


CHAP,  in.]  PACKING  OF  SOIL  PARTICLES  61 

size  of  the  spheres.  If  the  system  of  packing  shown  in 
A  and  B,  Fig.  2,  is  adopted,  the  pore  space  reaches  its 
maximum  and  amounts  to  47-64  per  cent  of  the  whole 
volume  occupied  by  the  soil ;  this  proportion  is  the  same 
when  the  soil  particles  have  a  smaller  diameter,  as  in  B  ; 
as  long  as  the  spheres  are  uniform  in  size,  whatever  that 
may  be,  and  are  packed  as  shown  in  the  diagram,  the 
pore  space  will  be  at  its  maximum.  The  minimum  pore 
space  is  attained  by  the  packing  shown  in  C  and  D ;  it 
amounts  to  25-95  Per  cent,  and  is  again  independent  of 
the  size  of  the  particles,  provided  they  are  uniform.  If 
the  spheres  are,  however,  of  very  different  sizes,  so  that 
smaller  spheres  lie  wholly  within  the  spaces  between 
the  larger  spheres,  as  in  the  arrangement  shown  in  E, 
the  pore  space  may  be  indefinitely  reduced.  Per  contra, 
if  aggregates  of  particles  exist  in  the  soil,  containing 
both  pore  space  between  the  ultimate  particles  and 
between  the  aggregates  which  behave  as  single  particles, 
as  in  F,  the  pore  space  may  rise  much  above  the 
maximum  of  49  per  cent  A  soil  in  situ  generally 
possesses  a  pore  space  larger  than  the  proportions 
indicated  above ;  various  causes,  such  as  the  stirring 
due  to  cultivation,  the  decay  of  vegetation,  etc.,  leave 
definite  cavities  in  the  soil :  for  example,  if  a  hole  be 
dug  for  any  purpose  in  ordinary  cultivated  ground  and 
afterwards  filled  up  with  its  own  soil,  it  is  rarely  possible 
to  fill  the  hole  completely,  especially  if  a  little  pressure 
has  been  used  to  trample  down  each  layer. 

In  ordinary  soils  the  pore  space  varies  from  a  little 
over  50  per  cent  among  the  stiff  clays,  down  to  25  or  30 
per  cent  in  the  case  of  coarse  sands  of  uniform  texture. 
The  reason  for  the  greater  pore  space  with  the  finer 
grained  soils  lies  in  the  fact  that  the  weight  of  the 
small  particles  of  clay  is  not  sufficient  to  overcome  the 
friction  and  move  the  particles  into  the  arrangement 


62 


THE  TEXTURE  OF  THE  SOIL 


[CHAP. 


III.] 


TRUE  AND  APPARENT  DENSITY 


giving  the  minimum  pore  space.  If  some  small  shot  are 
shaken  into  a  graduated  measure  and  the  pore  space 
determined  by  pouring  in  a  measured  volume  of  water, 
the  indicated  minimum  will  be  found ;  but  if  the  experi- 
ment be  repeated  with  sand  which  has  been  sifted  to  get 
approximately  a  uniform  size,  a  higher  figure  will  result 
In  the  one  case  the  particles  are  too  light  to  exert 
much  force  towards  the  rearrangement  of  the  mass ;  in 
the  former  case  the  heavy  smooth  shot  slip  straightway 
into  the  most  compact  arrangement,  because  by  it  the 
shot  attain  their  lowest  position.  In  consequence  of  the 
pore  space,  the  density  of  a  soil  in  situ  will  differ  very 
much  from  that  of  the  materials  of  which  it  is  composed, 
nor  will  all  soils  possess  the  same  apparent  density  when 
dry.  Perhaps  the  best  way  of  ascertaining  the  apparent 
density  of  a  soil  or  soil  materials  is  to  get  a  smooth 
metal  pint  pot  or  like  measure,  fill  it  with  the  material 
in  question  with  gentle  tapping,  and  then  strike  off  the 
upper  surface  smooth  with  a  rule.  The  weight  of  the 
contents  divided  by  the  volume  gives  the  apparent 
density,  from  which  the  true  volume  and  the  pore  space 
can  be  calculated,  if  the  true  density  of  the  material  be 
known.  The  following  table  shows  the  true  and  ap- 
parent density  of  the  chief  soil  materials ;  as  a  mean 
figure  for  purposes  of  calculation,  2-65  can  be  taken  as 
the  true  density  of  ordinary  soils  : — 


True  Density. 

Apparent  Density 
when  dry. 

Humus    ..... 
Clay        

1-2 

2-5 
2-6 

•34 
I 

I  -45 

Calcium  Carbonate  . 
Hydrated  Oxide  of  Iron  . 

2-75 
3-4  to  4 

The    following    table    shows   a  few   determinations 


64 


THE  TEXTURE  OF  THE  SOIL 


[CHAP. 


made  in  the  laboratory,  of  the  apparent  density  of 
various  soils  in  a  roughly  powdered  state  and  without 
the  stones,  which,  being  solid,  would  add  to  the  apparent 
density  of  the  soil.  The  results  are  also  recalculated 
to  show  the  weight  of  a  cubic  foot  of  the  soil,  and  the 
weight  per  acre  of  a  layer  9  inches  deep  : — 


Apparent 
Density. 

Weight  per 
cubic  foot. 

Lbs.  per  acre 
to  9". 

Heavy  Clay  . 

1062 

66-4 

2,150,000 

Sandy  Clay    . 

1-279 

80 

2,600,000 

Sandy  Clay  Subsoil 

I-I8 

73-7 

2,380,000 

Light  Loam  . 

1-222 

76-4 

2,480,000 

Light  Loam  Subsoil 

I-I44 

71-5 

2,32O,OOO 

Sandy  Loam  . 

1-225 

76-7 

2,490,000 

Sandy  Peat    . 

0-782 

49 

1,580,000 

Light  Sand    .         . 

1-266 

79-2 

2,560,000 

The  figures  given  above  are  not  exactly  comparable 
with  soils  under  natural  conditions,  because  of  the 
powdering,  the  exclusion  of  stones,  etc.,  but  they 
serve  to  show  that  the  clay  soils  usually  described  as 
"  heavy  "  are  really  less  dense,  and  weigh  less  per  cubic 
foot  than  some  of  the  lighter  soils,  whereas  pure  sands 
are  the  densest  of  all.  The  farmer's  terms  of  "  light " 
and  "  heavy "  land  refer  to  the  draught  of  the  plough, 
the  resistance  the  soil  opposes  to  being  torn  asunder, 
and  not  to  the  actual  weight  of  the  portion  moved ; 
sands  which  he  calls  "  light,"  being,  as  the  table  shows, 
heavier  per  cubic  foot  than  the  clays  which  the  farmer 
calls  heavy  soils. 

This  point  will  be  further  elucidated  by  the  following 
table,  which  shows  the  weight  per  cubic  foot  of  the 
arable  soils  at  Rothamsted  and  Woburn  down  to  a 
depth  of  3  feet  These  results  represent  the  real  weights 
of  the  soil  as  obtained  by  cutting  out  a  block  6  inches 
square  by  9  inches  deep,  weighing  it,  and  afterwards 


III.] 


WEIGHT  OF  SOIL 


ascertaining  the  deduction  to  be  made  for  water.  The 
Rothamsted  soil  is  a  stiff  clay  with  many  flints,  the 
Woburn  soil  is  a  loose,  coarse-grained  sand,  containing 
only  a  little  stone  derived  from  the  rock  below.  It  will 
be  seen  that,  if  the  stones  are  excluded,  the  density 
increases  with  the  depth,  because  of  the  greater  consoli- 
dation caused  by  the  weight  above  and  to  some  extent 
by  the  washing  down  of  the  finest  particles,  but  the 
increase  does  not  continue  much  below  the  depth  of  3 
feet,  the  limit  of  these  measurements  : — 


Weight  per 
cub.  foot. 

Per  cent, 
of  Stones. 

Weight  per 
acre. 

Lbs. 

Lbs. 

Jo"    to     9" 
9"           1  8" 

95-4 
93-0 

1  6-8 
12-0 

3,Il6,OOO 
3,037,000 

Broadbalk           I    18"           27" 

92-0 

7-i 

3,004,000 

I  27"           36" 

92-3 

7-5 

3,012,000 

j     °"            9" 

96-6 

2-96 

3,157,000 

Woburn  Arable      .  •[    $,           l*l 

103-8 
106-2 

5-95 
4-92 

3,382,000 
3,462,000 

[  27"           36" 

106-9 

7-83 

3,501,000 

From  the  data  thus  obtained  as  to  density  and 
pore  space,  together  with  a  mechanical  analysis  to 
show  the  proportion  of  particles  of  various  sizes,  it  is 
possible  to  calculate  for  any  given  soil  both  the  number 
of  soil  particles  and  the  area  of  the  surface  they  expose, 
on  the  assumption  that  the  particles  are  spherical. 
Approximately  with  grains  i  mm.  in  diameter,  there 
would  be  700  grains  in  I  gram  of  the  soil,  and  the 
number  of  grains  to  the  gram  will  vary  inversely  as 
the  third  power  of  the  diameter,  i.e.,  if  the  diameter  be 
divided  by  10  and  become  o-i  mm.,  there  will  then  be 
700,000  grains  to  the  gram.  The  surface  possessed  by 
all  the  soil  grains  can  be  similarly  deduced  by  calcula- 
tion, and  will  be  found  to  vary  inversely  as  the  diameter 

E 


66 


THE  TEXTURE  OF  THE  SOIL 


[CHAP. 


of  the  individual  grains ;  a  sphere  I  inch  in  diameter 
will  have  only  half  the  surface  of  the  eight  spheres  of 
half  an  inch  in  diameter  which  possess  the  same  volume. 
Hence  it  follows  that  the  surface  of  an  ordinary  soil 
must  be  extremely  extensive,  and  since  many  of 
the  properties  of  the  soil  are  dependent  upon  the  surface 
it  becomes  important  to  arrive  at  some  measure  of 
this  quantity.  By  calculation  only  a  very  rough  idea 
of  its  extent  can  be  formed,  both  because  every 
departure  of  the  soil  grains  from  the  spherical  form 
will  increase  the  surface  without  affecting  the  weight, 
and  also  because  the  mechanical  analysis  of  a  soil 
gives  only  a  generalised  statement  of  the  distribu- 
tion of  soil  particles  of  various  sizes  in  the  soil.  But 
the  surface  of  the  soil  grains  in  the  case  of  a  sandy 
soil  where  the  grains  are  all  free,  may  be  calculated 
from  the  observed  rates  of  flow  of  fluids  like  air  or 
water  through  a  measured  portion  of  the  sand ;  and  by 
using  this  method  King  has  computed  the  surface  of 
the  constituent  particles  of  various  types  of  soil  with 
the  results  set  out  below  : — 


Pore  Space,  per 
cent. 

Area  of  Surface  in 
square  feet, 
per  cubic  foot  of  Soil. 

Finest  Clay     . 

52-9 

173,700 

Fine  Clay  Soil 

48 

110,500 

Loamy  Clay  Soil               * 

49-2 

70,500 

Loam 

44-1 

46,500 

Sandy  Loam  . 

38-8 

36,900 

Sandy  Soil 

32-5 

II.OOO 

As  a  rough  figure  to  remember,  the  surface  of  the 
particles  in  one  cubic  foot  of  an  ordinary  light  loam 
may  be  taken  as  about  an  acre ;  this  will  increase 
as  the  soil  approaches  more  and  more  to  clay,  and 
diminish  as  the  soil  becomes  increasingly  sandy.  The 


III.]  WATER  IN  SATURATED  SOIL  67 

extent  of  surface  exposed  by  the  soil  particles  is  im- 
portant because  it  is  their  active  part ;  other  conditions 
being  equal,  the  amount  dissolved  from  a  solid  body  in 
a  given  time  by  any  solvent  will  be  proportional  to 
the  surface  exposed. 

Again,  the  water  in  a  soil  usually  exists  as  a  film, 
coating  the  surface  of  the  soil  particles,  and  the  amount 
of  water  that  can  be  held  under  particular  conditions 
becomes  a  function  of  the  extent  of  surface ;  even  the 
power  of  a  soil  to  remove  certain  substances  from  solution 
is  likewise  dependent  on  the  surface. 


Capacity  of  the  Soil  for  Water. 

So  far,  the  structure  of  the  soil  in  a  dry  state  has 
only  been  considered,  it  is  now  necessary  to  consider 
its  behaviour  when  fully  saturated  with  water,  before 
passing  on  to  the  more  usual  state  when  the  soil  con- 
tains both  air  and  water. 

The  amount  of  water  which  a  soil  will  hold  when  com- 
pletely saturated  will  depend  upon  the  pore  space,  will, 
in  fact,  be  the  pore  space  together  with  whatever  water 
the  material  of  the  particles  can  imbibe  without  causing 
any  swelling.  Perhaps  the  best  method  for  determining 
the  water  capacity  of  a  soil  is  one  devised  by  Hilgard. 
A  small  cylindrical  brass  box  is  constructed,  I  cm. 
deep  and  6  cm.  in  diameter.  The  bottom  is  a  sheet  of 
perforated  brass,  and  the  whole  is  supported  on  three 
legs  ;  the  capacity  of  the  box  is  about  30  c.c.  The 
exact  capacity  is  determined  by  waxing  up  the  holes, 
weighing,  filling  with  water,  and  reweighing.  A  circle 
of  thin  filter  paper  cut  to  fit  the  box  is  laid  inside 
and  wetted,  any  superfluous  water  that  comes  through 
being  wiped  away.  The  box  is  then  weighed,  care- 


68  THE  TEXTURE  OF  THE  SOIL  LCHAP- 

fully  filled  with  fine  earth,  and  gently  tapped  to  settle 
the  soil  down ;  finally,  the  surface  is  struck  off  level 
with  a  straight-edge.  The  box  is  now  weighed  again 
to  find  the  quantity  of  dry  soil  taken,  and  placed  in  a 
dish  of  distilled  water,  so  that  the  water  stands  about 
i  mm.  above  the  lower  surface  of  the  soil  inside  the 
box ;  the  dish  is  then  covered  over  to  prevent  evapora- 
tion. The  water  rises  in  the  soil,  displacing  the  air, 
and  in  about  an  hour's  time  the  soil  will  have  absorbed 
all  the  water  possible.  The  box  is  lifted  above  the  water 
a  little,  allowed  a  few  minutes  to  drain,  the  excess  of 
water  clinging  to  the  under-surface  is  wiped  away  with 
a  clean  cloth  or  filter  paper,  and  the  whole  is  then 
weighed.  A  previous  determination  of  the  moisture 
present  in  the  "  air-dry  fine  earth  "  must  also  be  made, 
to  provide  all  the  data  necessary  for  the  calculation  of 
the  water  contained  in  the  saturated  soil.  This  calcu- 
lation may  be  made  in  three  ways:  either  the  pro- 
portion the  water  in  the  saturated  soil  bears  to  the 
dry  soil,  or  the  proportion  of  water  in  the  wet  soil 
may  be  estimated,  or  again,  the  proportion  by  volume 
that  is  occupied  by  water  and  soil  respectively  may 
be  calculated.  The  figures  thus  obtained  will  vary  very 
considerably,  because  the  less  dense  the  soils,  because  of 
the  clay  and  humus  they  contain,  the  more  water  they 
will  absorb;  thus  the  proportion  which  the  water 
absorbed  bears  to  the  weight  of  the  dry  soil  becomes 
exaggerated  in  their  case.  Perhaps  the  soundest  picture 
of  the  state  of  affairs  is  attained  by  considering  the 
volume  that  is  occupied  by  the  water  in  the  soil,  and 
expressing  it  either  as  a  percentage  by  volume,  or  as 
Ibs.  or  inches  of  water  per  cubic  foot  of  wet  soil.  The 
following  figures  show  the  results  obtained  for  four 
distinctive  soils,  calculated  out  in  the  different  ways 
described  above. 


III.] 


WATER  RETAINED  BY  SOIL 


69 


MAXIMUM. 

MINIMI  M. 

fc 

d 

S3 

C  •— 

j 

£  33 

•6  QQ    / 

5^« 

1 

£» 

_fr 

J£ 

Sit 

^•g  g 
0-52 
^^ 

f 

!} 

=1 

11 

i£^ 

£"* 

fe  * 

tt  u 

e  " 

**  a 

5 

_& 

£ 

£* 

Coarse  Sandy  Soil 
Light  Loam  . 

45 

31 

33-5 

50-5 
55-8 

18 
29-2 

22-6 

22-2 

35-4 

0-8 

2^ 

Stiff  Clay      . 

98-6 

49.6 

67-6 

56-4 

36-1 

45-6 

6-9 

Sandy  Peat   . 

155 

60-8 

63-2 

116 

53-7 

52-8 

8-3 

Under  natural  conditions  a  soil  is  rarely  saturated 
to  the  extent  indicated  in  the  previous  table;  as  the 
rain  water  enters  from  above,  the  surface  of  the  soil 
is  wetted  first  and  the  air  within  the  soil  finds  a  diffi- 
culty in  escaping,  so  that  even  after  long-continued 
rain  the  pore  space  does  not  become  entirely  filled 
with  water. 

The  following  table  shows  the  water  contained  in 
a  few  field  soils  sampled  a  day  or  two  after  the  cessa- 
tion of  long-continued  rain,  and  calculated  as  per- 
centages of  the  wet  soil  by  weight : — 


Per  cent,  of 

Water 

in  Wet  Boil. 

Sand  at  Water  Level  . 

18-4 

Rothamsted  Wheat  Land 

unmanured         .... 

23-0 

)>                   i) 

manured  with  artificials 

24-7 

i)                   >i 

manured  with  dung  for  26  years 

37-6 

Light  Loam  above  Chulk 



20-3 

Hellriegel  has  shown  that  the  optimum  proportion 
of  water  in  the  soil  for  the  growth  of  the  plant  is 
40  to  50  per  cent  of  the  maximum  required  for  satura- 
tion. 


TEXTURE  of  THE  SOIL 


[CHAP. 


Flow  of  Water  through  Soils. 

The  freedom  with  which  water  will  move  through 
soils  under  the  action  of  gravity  or  other  force  will 
depend  not  only  on  the  pore  space,  but  upon  the 
mean  size  of  the  channels  formed  between  the  soil 
grains.  King  made  some  experiments  with  sands 
graded  by  sieves  and  formed  into  columns  14  inches 
long  and  I  square  foot  in  section,  above  which  the  water 
was  maintained  at  a  head  of  2  inches.  He  obtained 
the  following  results  expressed  in  inches  of  water 
passing  in  twenty-four  hours ;  the  second  column  gives 
the  number  of  meshes  to  the  inch  of  the  sieves  which 
respectively  passed  and  retained  the  sand  : — 


Medium. 

Sieves. 

Inches. 

Sand  

40  to   60 
60  „     80 

301 
160 

" 

80  „  loo 

77.3 

IOO 

•jq.7 

Clay  Loam 
Black  Marsh  Soil 

1-6 

•7 

It  will  be  noticed  that  there  is  a  great  diminution 
in  the  rate  of  flow  as  soon  as  a  soil  containing  small 
clay  particles  is  introduced ;  of  course,  one  of  the 
characteristic  properties  of  clay  is  that  it  will  not 
allow  any  flow  of  water  through  it  when  it  has  been 
puddled.  In  the  puddled  condition,  the  particles 
constituting  the  clay  are  no  longer  aggregated,  the 
material  is  in  its  finest-grained  condition,  so  that  the 
pore  spaces  between  them  must  have  become  extremely 
small.  Not  only  is  the  flow  diminished  by  the  increase 
of  friction  in  the  narrow  channels,  but  in  the  case  of 
clay  their  dimensions  have  become  so  small  that  prob- 
ably the  contained  water  is  wholly  within  the  range 


HI.]  SURFACE  TENSION  71 

of  the  molecular  forces  to  be  described  later ;  it  is  thus 
prevented  from  flowing  at  all,  and  only  moves  by 
diffusion.  If  we  assume  for  clay  particles  a  mean 
diameter  of  00002  mm.,  and  a  structure  similar  to  A  in 
Fig.  2,  p.  62,  it  is  easy  to  show  that  no  molecule  in  the 
space  between  the  spheres  can  be  further  than  about  £ 
of  the  diameter  of  a  sphere,  or  0-00004  mm-  from  one  or 
other  surface,  while  the  range  of  molecular  forces  as 
calculated  by  Quincke  extends  to  about  0-00005  mm- 
from  the  surface.  Spring  has  indeed  shown  that  infiltra- 
tion of  water  is  impossible  through  clays  or  loams  unless 
they  are  first  allowed  to  expand  by  taking  up  water. 

Surface  Tension  and  Capillarity. 

The  existence  of  attraction  between  the  molecules 
causes  the  free  surface  of  any  liquid  to  become  a  sort 
of  stretched  elastic  film,  in  tension  itself,  and  exerting 
a  certain  pressure  inwards  when  free.  The  molecules 
within  the  liquid  are  equally  attracted  in  all  directions 
by  the  surrounding  molecules,  and  are  therefore  in  equili- 
brium ;  the  molecules  on  the  surface,  having  nothing  on 
one  side,  are  only  attracted  inwards,  and  so,  as  a  whole, 
exert  a  pressure  on  the  liquid  similar  to  that  which 
would  be  caused  by  a  stretched  elastic  skin  over  the 
liquid. 

The  existence  of  this  force  of  "  surface  tension,"  as 
it  is  called,  may  be  demonstrated  by  many  simple  ex- 
periments, e.g.,  by  the  familiar  fact  that  a  clean  needle 
will  float  when  placed  carefully  on  the  surface  of  water  ; 
or,  by  the  fact  that  any  portion  of  a  liquid  which  is  so 
small  that  the  force  of  gravity  on  it  is  not  large 
compared  to  the  molecular  forces,  immediately  assumes 
the  spherical  shape.  Of  all  figures,  a  sphere  has  the 
smallest  surface  in  proportion  to  its  contents,  i.e.,  the 


THE  TEXTURE  OF  THE  SOIL 


[CHAP. 


stretched  film  on  the  surface  of  a  drop  of  liquid  shrinks 
as  far  as  it  can  until  the  liquid  is  packed  into  the 
smallest  possible  compass,  into  the  form  of  a  sphere. 

When  a  liquid  and  a  solid  are  in  contact,  the  form  of 
the  surface  and  the  resulting  pressure  or  tension  depend 
on  whether  the  liquid  "wets"  the  solid  or  not  For 


FlG.  3. — Capillary  Rise  and  Depression 
of  Liquids  in  Glass  Tubes. 

example,  if  a  series  of  very  fine  or  "  capillary  "  glass 
tubes  are  dipped  into  water  and  mercury  respectively, 
the  water  will  rise  up  the  tubes  in  inverse  proportion  to 
their  diameters,  the  mercury,  which  does  not  wet  the 
glass,  will  be  correspondingly  depressed. 

The  water  surfaces  a,  b,  c  (Fig.  3),  are  convex  to 
the  water,  and  become  more  convex  the  narrower  the 
tube  is ;  the  pressure  below  the  convex  surface  must 


Fl  ;.  a. — Photograph  illustrating  Liquid   Film   round  Soil   Particles. 


[To  face  page  73. 


HI.]  SURFACE  TENSION  73 

be  less  than  atmospheric,  or  the  water  would  not 
stand  higher  within  than  without  the  tube;  further, 
the  pressure  beneath  a,  the  most  convex  and  therefore 
most  stretched  surface  film,  is  lower  than  the  pressure 
beneath  b,  and  still  lower  than  that  beneath  c.  Per 
contra,  the  mercury  surfaces  are  convex  outwards,  and 
exert  pressure  on  the  liquid  beneath,  depressing  it 
below  the  general  surface  of  the  liquid  in  proportion 
to  the  degree  of  convexity.  These  instances  will  help 
us  to  realise  that  the  surface  of  a  liquid  may  exert 
either  a  pull  or  a  pressure  on  the  liquid  within, 
according  to  the  curvature  of  the  surface,  and  the 
greater  the  curvature  the  greater  will  be  the  force 
exerted.  It  is  this  tension  of  the  surface  film  which 
causes  movements  of  water  in  soil,  other  than  those  due 
to  gravity ;  for  example,  if  a  flowerpot  stands  in  a 
shallow  dish  of  water,  the  whole  of  the  soil  within  the 
pot  is  kept  moist ;  or  if  water  is  poured  on  to  dry  soil,  it 
is  seen  to  work  outwards  through  the  soil,  the  water 
advancing  from  particle  to  particle  as  it  wets  them,  just 
in  the  same  manner  as  it  rises  up  the  capillary  tubes. 
When  a  soil  is  saturated,  the  whole  pore  space  is  filled 
with  water ;  if  this  soil  be  allowed  to  drain,  some  of  the 
water  is  pulled  away  by  gravity,  but  much  remains 
clinging  round  the  particles  in  the  stretched  film  con- 
dition, the  tension  in  the  film  balancing  the  pull  due  to 
gravity.  Perhaps  the  best  illustration  of  the  state  of 
affairs  in  a  wet  but  drained  soil  may  be  obtained  by 
linking  a  series  of  toy  balls  together,  as  shown  in  the 
photograph  (Fig.  4),  and  then  dipping  the  whole  into  oil. 
When  the  oil  has  ceased  to  drip  it  will  be  seen  that 
every  ball  is  covered  by  a  thin  film  of  oil,  and  that 
between  the  balls  there  is  a  layer  of  oil  much  thicker  in 
the  lower  than  in  the  upper  layers.  The  whole  surface 
film  is  equally  stretched,  but  the  stretching  in  the  upper 


74  THE  TEXTURE  OF  THE  SOIL  [CHAP. 

layers  is  largely  due  to  the  pull  from  the  oil  below, 
while  in  the  lowest  layer  of  all  the  whole  tension 
exerted  by  the  stretched  film  is  devoted  to  holding  up 
its  own  thick  film  of  oil.  If  oil  be  taken  away  at  any 
point,  the  curvature  of  the  film,  and  therefore  the  tension 
of  the  surface  in  that  region,  is  increased :  a  readjust- 
ment then  takes  place  till  the  stretched  film  regains  the 
same  tension  everywhere,  which  is  effected  by  a  motion 
of  the  oil  to  the  place  where  the  tension  has  been 
increased.  If  the  withdrawal  of  the  oil  be  continued,  the 
film  round  the  balls  becomes  thinner  and  thinner;  the 
more  it  is  stretched,  the  more  closely  it  clings  to  the 
surface,  so  that  the  removal  becomes  progressively  more 
difficult ;  at  last  the  film  becomes  so  much  stretched 
that  it  ruptures  and  reunites  again  over  a  smaller 
surface,  hence  with  a  diminished  tension.  The  rupture 
naturally  takes  place  where  the  film  is  thinnest,  on  the 
top  layer  of  balls,  which  becomes  more  or  less  "dry" 
while  the  lower  balls  are  still  surrounded  by  their 
film. 

Just  in  a  similar  way  water  will  always  move  in  a 
soil  from  a  wet  to  a  dryer  place,  till  the  film  surround- 
ing the  particles  is  equally  stretched  throughout. 

For  example,  if  A,  B,  C  (Fig.  5)  represent  three  soil 
particles,  of  which  A  and  B  are  surrounded  by  a  thin, 
and  C  by  a  thicker,  film  of  water :  when  the  spheres 
are  in  contact  the  water  will  fill  up  part  of  the  angle 
between  the  spheres,  as  shown  in  the  diagram.  But 
the  water  surface  at  a  is  more  curved  than  at  b,  i.e., 
it  corresponds  to  the  surface  at  a  in  the  fine  capillary 
tube  (Fig.  3)  as  compared  with  the  surface  at  b  in  the 
wider  tube.  But  the  diminution  of  pressure  caused  by 
a  is  greater  than  that  caused  by  b,  as  shown  by  the 
greater  height  to  which  water  is  raised  in  the  tube ; 
hence  in  the  same  way  the  pressure  inside  the  liquid 


III.]  SURFACE  TENSION  75 

at  a  (Fig.  5)  will  be  lower  than  that  at  b,  and  there 
will  be  a  flow  of  water  from  b  to  a,  until  the  curva- 
tures and  corresponding  surface  tensions  are  equalised. 
In  a  wet  soil,  then,  surface  tension  is  a  force  tending 
on  the  one  hand  to  retain  a  certain  amount  of  water 
round  the  particles,  and  on  the  other  to  equalise  the 
distribution  of  water,  by  causing  movement  towards  any 
point  where  the  surface  tension  has  been  increased. 
For  example,  if  the  water  in  a  soil  is  in  equilibrium  and 
evaporation  begins  at  the  surface,  the  film  there  is  made 


FlG.  5. — Diagram  illustrating  Liquid  Film  round  Soil  Particles. 

thinner,  and  the  curvature  increased  in  the  angles 
between  the  soil  particles :  hence  the  pull  exerted  by 
the  film  is  increased,  and  water  is  lifted  from  below 
against  gravity.  Per  contra,  if  rain  fall  on  such  a  soil 
the  films  round  the  upper  particles  are  thickened,  their 
tension  is  lowered,  and  the  pull  of  the  film  below  now 
acts  with  gravity  in  drawing  the  water  down  into  the 
soil. 

Percolation. 

The  state  of  affairs  illustrated  by  the  model  of  balls 
dipped  in  oil  is  seen  in  the  case  of  a  soil  \vhich  has  been 
thoroughly  saturated  so  that  all  the  pore  space  is  occu- 
pied by  water,  and  then  allowed  to  drain  until  the  remain- 
ing water  is  held  in  the  soil  by  surface  tension  only. 


THE  TEXTURE  OF  THE  SOIL 


[CHAP. 


In  the  upper  layers  the  film  will  be  stretched  to  the 
utmost,  or  even  broken  by  the  pull  of  the  water  below  ; 
in  the  lower  layers  the  film  will  be  wholly  engaged  in 
holding  the  water  immediately  in  contact  with  the 
particles  of  the  layer:  these  layers  may  be  saturated, 
while  the  upper  layers  hold  an  amount  dependent  on 
their  distance  from  the  saturated  zone,  and  on  the 
extent  of  surface  exposed  by  the  particles. 

The  accompanying  diagram  (Fig.  6)  expresses  the 
results  of  an  experiment  of  King's,  where  columns  of 
sand  and  soil,  8  feet  and  7  feet  long  respectively,  were 
saturated  and  then  allowed  to  drain  till  they  parted 
with  no  further  water,  which  required  a  period  of  sixty 
days  in  the  case  of  the  soil  columns,  and  of  more 
than  two  years  for  the  sand.  The  tubes  were  then 
cut  up,  and  the  proportion  of  water  in  the  sand  or 
soil  in  successive  3-inch  lengths  of  the  tubes  was 
determined 

It  will  be  seen  that  the  sands  retain  very  little  water 
by  surface  tension  in  the  upper  layers,  whereas  the  clay 
loam,  with  the  enormous  area  its  particles  expose,  holds 
practically  the  same  proportion  throughout. 

If  we  also  consider  the  following  table,  showing  the 
time  taken  by  the  same  sands  and  soils  to  part  with 
their  water,  the  difference  of  the  texture  of  the  soils  will 
be  even  more  evident : — 


INCHES  OF  WATER  LOST  IN 

SO  min. 

81  to  60 

min. 

24  hrs.  (?) 

2  to  11 
days. 

12  to  21 
days. 

No.    20  Sand 

10-25 

4-68 

„      60     „ 

5-67 

4.52 

it     IOO       n 

I-2I 

.84 

Sandy  Loam 

2-64 

5-07 

•9 

Clay  Loam 

... 

1-96 

2-II 

•49 

FlG.  6. — Water  Content  of  Columns  of  wetted  but  thoroughly  drained 
Sand  and  Soil. 


[To  /ace  page  7(5. 


HI.]  PERCOLATION  OF  WATER  77 

The  downward  movement  of  rain  water  through 
soils  is  known  as  "  percolation,"  and  is  distinguished  from 
"  flow "  by  the  fact  that  the  water  is  supposed  to  have 
free  surfaces,  so  that  surface  tension  comes  into  play. 
It  takes  place  under  the  action  of  gravity  through  the 
pore  space  proper,  and  also  through  the  cracks,  the 
worm  tracks,  the  passages  left  by  decayed  roots,  and 
other  adventitious  openings  in  the  soil.  The  percola- 
tion proceeds  until  the  zone  is  reached  where  the  pore 
space  is  completely  filled ;  this  is  known  as  the  "  water 
table,"  and  is  the  level  at  which  water  stands  in  the 
wells.  Above  the  water  table  the  soil  will  be  more  or 
less  in  the  state  represented  in  the  diagram  showing 
sands  and  soils  in  which  percolation  has  ceased  ;  though 
there  will  be  most  probably  a  more  irregular  distri- 
bution, with  zones  which  contain  an  excess  of  water 
travelling  downwards  with  greater  or  less  rapidity, 
according  to  the  texture  of  the  soil.  It  is  these 
temporarily  saturated  zones  which  cause  the  ordinary 
tile  drains  to  run,  although  situated  many  feet  above 
the  permanent  water  table.  A  soil  in  which  percola- 
tion has  ceased,  though  it  may  still  contain  much 
water,  will  not  part  with  it  to  a  drain ;  the  water 
cannot  break  away  from  the  elastic  film  and  run 
off  down  the  drain,  unless  it  be  present  in  such  an 
excess  that  the  surface  tension  is  insufficient  to  hold 
it  against  gravity.  But  in  a  clay  soil,  percolation  is 
so  slow  that  the  upper  few  feet  of  soil  may  become 
saturated  by  the  winter  rains  and  remain  so  for 
months,  if  percolation  has  to  proceed  all  the  way  down 
to  the  water  table ;  by  the  introduction  of  drains,  the 
percolating  column  is  shortened  to  the  distance  between 
the  surface  and  the  drain.  In  a  coarse-grained  sandy 
soil  percolation  is  very  rapid,  the  land  dries  quickly 
after  rain,  and  retains  a  minimum  of  water  by  surface 


78  THE  TEXTURE  OF  THE  SOIL  [CHAP. 

tension ;  in  fine-grained  soils,  which  are,  however,  not 
too  fine  for  percolation,  the  excess  of  rain  will  be 
removed  rapidly  enough  to  keep  the  soil  below  the 
saturated  condition,  yet  enough  water  may  be  retained 
to  supply  the  needs  of  the  crop  between  the  intervals  of 
rain. 

While  the  flow  of  water  from  a  field-drain  may  be 
taken  as  a  rough  measure  of  the  amount  of  percolation 
going  on  at  any  given  time  for  that  soil,  the  movement 
may  be  followed  more  closely  by  means  of  a  lysimeter 
or  drain-gauge.  As  a  rule,  the  records  of  these  instru- 
ments are  vitiated  by  the  disturbance  undergone  by  the 
soil  in  filling  them  ;  but  the  drain-gauges  at  Rothamsted 
were  constructed  by  building  cemented  walls  round 
blocks  of  earth  in  situ,  and  then  gradually  introducing  a 
perforated  iron  plate  below  to  carry  the  soil.  The 
following  diagram  (Fig.  7)  shows  the  mean  monthly 
records  of  rainfall  and  percolation  through  a  depth  of 
20  inches  and  60  inches  respectively,  over  a  period  of 
thirty-five  years. 

It  will  be  seen  that  of  the  total  rainfall  a  little  less 
than  one-half  percolates  through  60  inches  of  the 
Rothamsted  soil ;  it  should  be  remembered,  however, 
that  the  surface  of  these  gauges  is  kept  free  from 
weed  or  any  growth.  The  total  drainage  through 
20  inches  of  soil  is  practically  the  same  as  that  through 
60  inches,  but  rather  a  greater  proportion  of  the  rainfall 
comes  through  the  6o-inch  gauge  in  the  winter  and 
through  the  2O-inch  gauge  in  the  summer.  In  the 
winter  months  the  percolation  reaches  as  much  as 
80  per  cent,  of  the  rainfall,  in  August  little  more  than 
20  per  cent,  of  the  rainfall  finds  its  way  through  the 
layer  of  soil.  When  the  ground  has  become  dried 
to  any  depth  in  the  summer,  percolation  may  be  much 
hindered  by  the  air  within  the  soil  and  the  want  of  a 


[To  fa 


HI.]  PERCOLATION  OF  WATER  79 

continuous  film  of  wetted  surfaces  to  lead  the  water 
down  by  surface  tension.  The  top  layer  of  soil  becomes 
thoroughly  wetted  and  will  not  allow  the  air  below  to 
escape ;  only  after  some  time  are  local  displacements  of 
the  air  set  up,  which  enable  the  water  above  to  make 
connection  with  the  wetted  subsoil  below,  so  that 
percolation  can  begin.  For  this  reason  summer  rains 
falling  in  a  season  of  drought  are  often  noticed  to  be 
of  little  benefit  to  the  crop,  because  they  are  retained 
near  the  surface  until  wholly  evaporated,  instead  of 
increasing  the  stock  of  moisture  at  the  lower  levels 
where  the  roots  of  the  plant  are  then  operative  in 
obtaining  water. 

Minimum  Capacity  of  Soil  for  Water. 

The  amount  of  water  retained  in  a  soil  by  surface 
tension  alone,  when  percolation  has  removed  as  much 
as  possible,  is  rather  an  important  factor  to  determine, 
as  upon  it  depends  to  some  extent  the  power  of  the 
soil  to  resist  drought  by  retaining  water  for  the  crop 
between  intervals  of  rain.  If  short  columns  of  sand  and 
soil  which  have  been  saturated  and  then  allowed  to 
drain  away  into  a  state  of  equilibrium  are  considered, 
it  is  clear  that  very  different  proportions  of  water 
are  retained  by  the  various  soils.  Suppose,  however, 
the  column  be  of  such  a  length  that  at  some  level  the 
upper  film  of  water  cannot  be  further  stretched,  but 
the  particles  cease  to  be  wet;  the  layer  immediately 
below  this  dry  soil  contains  the  minimum  amount  of 
water  consistent  with  a  continuous  film  at  all.  Soil 
in  this  condition  may  be  regarded  as  at  the  minimum 
of  saturation ;  it  will  part  with  no  more  water  by 
drainage,  and  will  become  drier  only  by  evaporation. 
In  order  to  obtain  such  a  sample  of  soil  for  determina- 


8o  THE  TEXTURE  OF  THE  SOIL  [CHAP. 

tion  of  its  water  content,  much  trouble  would  be 
necessary,  and  a  long  time  must  elapse  before  equili- 
brium could  be  obtained  in  the  long  tube  filled  with 
wet  soil.  Practically,  however,  the  same  result  can 
be  reached  with  the  apparatus  previously  described 
for  estimating  the  maximum  capacity  of  the  soil  for 
water,  by  constantly  bringing  the  thin  layer  of  soil 
there  used  into  contact  with  dry  soil,  until  the 
previously  saturated  soil  no  longer  parts  with  water 
to  the  new  soil. 

After  the  determination  of  maximum  water  capacity, 
as  previously  described  (p.  67),  a  little  more  fine  earth, 
which  has  been  standing  for  some  time  over  water  in  a 
closed  space  so  that  it  has  acquired  all  the  hygroscopic 
moisture  it  can,  is  shaken  lightly  over  the  surface  of  the 
wet  soil  in  the  box  to  the  depth  of  \  inch  or  so.  It 
rapidly  becomes  wet,  as  will  be  evident  by  a  change  in 
colour,  whereupon  it  is  struck  off  by  drawing  a  fine 
tightly  stretched  wire  across  the  top  of  the  box  and 
shaking  the  loosened  layer  off.  More  fine  earth  is  then 
shaken  on  and  struck  off  as  before  when  wetted,  the 
operation  being  repeated  again  and  again,  until  a  thin 
dry  layer  remains  on  the  surface  for  half  an  hour  or  so 
without  showing  by  change  of  colour  any  absorption  of 
water.  During  this  wait  the  box  should  be  in  a  closed 
chamber  over  water.  With  fine-grained  clays  and  loams 
the  process  does  not  take  long,  with  a  coarse  sand  the 
water  moves  slowly  into  the  dry  layer,  and  it  is  difficult 
to  hit  off  the  exact  end-point,  when  the  soil  particles 
are  still  surrounded  by  water  but  the  surface  tension 
is  too  great  to  allow  this  water  to  pass  into  dry  soil. 
Finally,  the  box  and  its  contents  are  weighed,  dried 
in  the  oven,  and  reweighed.  The  second  weighing 
when  dry  is  necessary,  because  the  box  will  be  found 
to  hold  more  dry  soil  than  was  originally  filled  into 


in.]        WATER  LIFTED  BY  SURFACE  TENSION       81 

it ;  a  certain  amount  of  consolidation  has  taken  place 
through  the  addition  of  the  dry  earth  and  the  subsequent 
striking  off  when  wet.  The  numbers  in  the  columns 
headed  Minimum  in  the  table  on  p.  69  were  obtained 
in  this  way :  they  show  that  though  the  maximum 
capacity  for  water,  or  pore  space,  may  not  vary  very 
greatly  for  different  soils,  there  is  a  much  wider  and 
more  important  divergence  between  the  amounts  of 
water  they  will  retain  by  surface  tension  alone,  this 
latter  being  the  important  factor  in  judging  of  the  power 
of  the  soil  to  retain  a  reserve  of  moisture  for  crops. 

Variations  in  Surface  Tension. 

The  surface  tension  of  water  is  very  high,  but  it  is 
easily  raised  or  lowered  by  the  presence  of  small 
amounts  of  material  in  solution.  The  effect  of  altering 
the  surface  tension  of  a  film  at  any  point  is  to  cause 
motion,  as  is  seen  in  the  well-known  experiment  of 
covering  a  plate  with  a  thin  film  of  coloured  water  and 
dropping  a  little  alcohol  into  the  middle  of  the  film. 
The  alcohol  immediately  weakens  the  surface  tension  of 
the  film  in  the  middle  to  such  an  extent  that  all  the 
liquid  runs  to  the  outer  edge  and  leaves  the  plate  bare 
in  the  middle.  Most  of  the  salts  which  are  used  as 
artificial  manures  and  are  soluble  in  water  increase  the 
surface  tension  of  the  soil  water,  hence  an  application  of 
salt  or  nitrate  of  soda  may,  by  increasing  the  tension  of 
the  surface  film,  lift  more  water  from  the  subsoil  and 
maintain  the  top  layer  of  soil  in  a  moister  condition. 
Per  contra,  solutions  of  organic  matter,  particularly  of  the 
many  organic  substances  used  as  manure  which  have  a 
little  oil  in  them,  extracts  of  dung,  etc.,  have  a  surface 
tension  below  that  of  water.  To  this  fact  may  be 
attributed  the  "  burning "  of  soils  which  is  sometimes 

F 


THE  TEXTURE  OF  THE  SOIL 


[CHAP. 


seen  when  organic  manures  are  applied  late  in  the 
season  and  dry  hot  weather  succeeds ;  the  soil  water  at 
the  top  in  contact  with  the  manure  has  a  lower  surface 
tension  and  consequently  less  lifting  power  for  the 
subsoil  water ;  hence  any  shallow  rooted  crop  is  deprived 
of  some  of  the  subsoil  water  which  would  have  otherwise 
been  lifted  to  it  A  rise  of  temperature  diminishes  the 
surface  tension  of  water,  and  therefore  lessens  the 
sustaining  power  of  the  film ;  as  it  also  lessens  the 
viscosity  of  water,  it  will  often  cause  percolation  to 
begin  afresh  from  soil  that  had  apparently  ceased  to 
yield  any  more  drainage.  This  effect  may  sometimes 
be  seen  in  variations  in  the  flow  of  land  drains  or  in  the 
level  of  water  in  shallow  wells.  The  following  table 
shows  the  comparative  surface  tension  of  water  and 
various  solutions : — 


Nature  of  Solution. 

Density. 

Surface  Tension. 

Water  . 
Common  Salt 
Kainit  . 
Nitrate  of  Soda 

10 
i-l 
I-I 
I-I 
1-0013 

Dynes  per  sq.  cm. 
7-532 
7-9II 
7-9 
7-73 

7  .4.6.1 

Superphosphate 
Soil  Extract  . 
Garden  Soil  Extract 

10104 

10 
10 

7.414 

7-244 
7089 

Cohesion  caused  by  Surface  Tension. 

In  certain  cases  the  stretched  film  surrounding  soil 
particles  will  give  them  an  apparent  cohesion  by 
enclosing  them  and  drawing  them  together.  A  handful 
of  wet  sand  can  be  moulded  into  shape,  but  falls  in 
pieces  as  soon  as  it  is  dry  :  just  as  in  a  camel-hair 
pencil  the  bristles,  which  stand  apart  when  dry  or 
wholly  immersed  in  water,  are  drawn  together  to  a 


HI.]        COHESION  DUE  TO  SURFACE  TENSION        83 

point  if  the  brush  be  dipped  in  water  and  withdrawn. 
Or  again,  a  flat  sandy  beach  from  which  a  smooth  sea 
is  receding  will  often  show  above  tide-mark  one  stretch 
of  sand  quite  dry  and  loose,  in  which  the  feet  sink 
deeply,  and  another  very  soft  stretch  immediately  left  by 
the  tide  where  the  sand  grains  are  completely  sur- 
rounded by  water.  Between  the  two  is  a  stretch  of 
sand  of  the  same  character,  but  firm  to  walk  upon ; 
this  is  partly  wet,  and  there  is  enough  water  to  form  a 
film  round  the  grains  and  hold  them  in  position  with  a 
certain  amount  of  force.  That  this  sand  is  really  just 
as  loosely  arranged  as  the  softer  tracts  that  are  either 
wetter  or  drier,  may  be  seen  by  the  fact  that  it  will 
easily  pack  more  closely  together  under  repeated  gentle 
pressure  with  the  foot.  The  shrinkage  of  soils,  especially 
of  clays,  as  they  dry,  may  be  attributed  to  the  surface 
tension  of  the  films  surrounding  the  groups  of  soil 
particles ;  as  the  water  content  is  lessened  the  films 
exert  more  force  in  their  effort  to  contract,  and  drag 
some  of  the  particles  closer  together,  especially  the  very 
small  particles  whose  weight  is  trivial  compared  to  the 
forces  exerted  by  the  film.  Clay  shrinks  more  than 
other  soils  because  of  the  greater  number  of  particles, 
their  small  size,  and  the  higher  proportion  of  pore  space 
into  which  motion  can  take  place.  The  tenacity  of  wet 
clay  is  due  to  the  number  of  water  films  that  have  to  be 
ruptured,  the  vastly  greater  cohesion  of  dry  clay 
probably  to  the  fact  that  many  of  the  particles  have 
been  dragged  within  the  range  of  one  another's 
molecular  forces.  There  is  a  stage  in  the  drying  of 
clay  when  it  will  fall  to  pieces  when  worked  ;  probably 
this  represents  the  stage  analogous  to  the  partly  wet 
sand,  when  cohesion  is  due  to  the  surface  films.  The 
clay  is  neither  so  wet  that  the  particles  just  slip  over 
one  another  when  pressure  is  applied — the  pasty 


84  THE  TEXTURE  OF  THE  SOIL  [CHAP. 

condition ;  nor  have  they  been  drawn  so  closely 
together  as  to  cohere  without  the  aid  of  any  water. 
It  is  not  quite  intelligible  why  a  piece  of  dried  clay 
becomes  soft  and  swells  again  when  wetted,  nor 
why  the  particles  should  once  more  move  apart. 


Hygroscopic  Moisture. 

If  the  withdrawal  of  water  from  a  soil  by  evapora- 
tion be  continued,  a  point  is  at  last  reached  when  the 
soil  becomes  air-dry  :  it  still  retains  some  water,  which 
will  vary  in  amount  with  the  degree  of  humidity  of  the 
atmosphere  and  the  temperature.  This  last  film  of 
water  is  held  very  closely  and  in  a  somewhat  different 
manner  from  the  ordinary  film  held  by  surface  tension, 
though  the  two  shade  off  into  one  another.  For  example, 
the  film  of  hygroscopic  moisture  can  be  produced  by 
condensation  alone,  when  perfectly  dry  soil  is  placed  in 
an  atmosphere  containing  water  vapour :  the  surface 
of  materials  like  glass,  sand,  etc.,  has  sufficient  attrac- 
tion for  water  to  condense  it  from  a  state  of  vapour. 
The  amount  of  hygroscopic  water  retained  by  different 
types  of  soil  when  air-dried  and  then  allowed  to 
stand  in  a  saturated  atmosphere  at  ordinary  tempera- 
ture, is  given  in  the  table  below ;  it  will  be  seen  to 
be  more  or  less  proportional  to  the  surface  possessed 
by  the  soil  particles,  clay  and  humus  retaining  the 
most. 

This  hygroscopic  moisture  cannot  be  of  any  service 
to  plants :  Sachs  has  shown  by  experiments  in  pots  that 
tobacco  plants  will  begin  to  wilt  before  the  soil  has 
parted  with  all  its  moisture.  When  wilting  began  with 
a  sandy  soil  the  sand  in  the  pot  still  contained  1-5  per 
cent,  of  water,  with  a  clay  soil  8  per  cent,  of  water ;  with 
a  mixture  of  sand  and  humus  there  was  as  much  as 


HI.] 


HYGROSCOPIC  MOISTURE 


12-3  per  cent,  of  water  retained  by  the  soil.  Heinrich 
has  further  shown  that  wilting  begins  before  the  water 
content  of  the  soil  has  been  reduced  to  the  hygroscopic 
water  limit,  as  the  following  figures  demonstrate — 


WATER  PER  l<> 

)  OK  DRV  SOIL. 

When  Plants  Wilt. 

Hygroscopic  Water. 

Coarse  Sand 

1-5 

MS 

Sandy  Garden  Soil 
Fine  Sand,  with  Humus 

4-6 
6-2 

3 
3-98 

Sandy  Loam 

7.8 

5-74 

Chalky  Loam     . 

9-8 

5-2 

Peat  . 

49-7 

42-3 

This  is  easily  intelligible  in  view  of  the  fact  that  the 
root  hairs  cannot  be  in  contact  with  all  the  soil  particles, 
nor  can  the  water  move  from  particle  to  particle  when 
it  has  been  reduced  to  so  low  a  proportion.  King  has 
made  some  observations  of  the  amount  of  water  still 
present  in  soils  in  the  field  which  had  become  so  dry 
that  growth  was  at  a  standstill  and  the  plants  were 
wilting. 


Depth. 

Nature  of  Soil. 

Per  cent,  of  Water 
under  Clover. 

Under  Maize. 

o"  to     6" 
6"   „    12" 
12"  „    1  8" 

Clay  Loam 
Reddish  Clay 

•& 

12-4 

b 

1  1  -6 

if 

18"   „   24" 

it 

13-3 

T2 

I1?, 

24"   ii    30" 

Sandy  Clay 

'13-5 

10-8 

1   'J    t 

40"   „   43" 

Sand 

9-5 

4-2 

Under  these  conditions  all  the  soils  were  about 
equally  dry,  so  far  as  any  power  to  part  with  their 
water  went ;  if  the  estimates  made  previously  of  the 
area  of  surface  of  the  particles  constituting  various  kinds 


86  THE  TEXTURE  OF  THE  SOIL  [CHAP. 

of  soil  be  combined  with  these  percentages  to  calculate 
the  thickness  of  film  of  the  water  on  that  surface,  it  will 
be  found  that  on  all  soils  the  film  possesses  approxi- 
mately the  same  thickness,  about  0-00003  inch. 

Because  of  the  quantity  of  water  which  some  soils 
will  retain  rather  than  give  up  to  the  plant,  it  is  possible 
that  such  soils  may  have  less  available  water  for  the 
plant  than  a  much  coarser  grained  soil  which  starts 
with  a  lower  initial  amount  of  water.  For  example,  the 
clay  and  sand  in  the  table  above  contain  when  satu- 
rated about  26  per  cent,  and  18  per  cent,  of  water 
respectively ;  as  the  crop  can  reduce  this  to  12  per  cent, 
in  one  case,  and  4-2  per  cent,  in  the  other,  both  sand 
and  clay  yield  about  the  same  amount  of  water  to  the 
crop. 

A  good  example  of  the  fact  that  only  the  water  in 
the  soil  which  is  in  excess  of  the  hygroscopic  moisture 
is  available  for  the  crop,  is  seen  in  F.  J.  Alway's  studies 
of  soil  moisture  conditions  in  the  "Great  Plains"  region 
of  north-western  America.  There  the  rainfall  is  only 
from  12  to  15  inches  annually,  and  falls  chiefly  during 
the  summer  months ;  because  of  its  insufficiency  for  the 
production  of  continuous  crops,  it  is  customary  to  take 
a  bare  fallow  one  season  in  three  in  order  to  accumulate 
the  rainfall  for  the  benefit  of  the  two  succeeding  grain 
crops. 

The  table  shows  the  water  content  of  the  soil  down 
to  the  depth  of  6  feet  on  two  fields  near  Indian  Head, 
Saskatchewan,  taken  at  the  end  of  July  1904,  field  B. 
having  been  fallowed,  and  C.  having  carried  a  crop  of 
oats  which  had  shown  the  effects  of  drought.  Figures 
are  given  for  the  total  water  in  the  soil  as  sampled,  the 
hygroscopic  moisture  as  determined  in  the  laboratory, 
and  the  difference,  which  may  be  termed  the  free 
water : — 


III.] 


HYGROSCOPIC  MOISTURE 


13,  A  M  KB  FALLOW. 

C,  AKTKR  OATS. 

WATER. 

WATBB. 

Total. 

Hygro- 
scopic. 

Free. 

Total. 

Hygro- 
scopic. 

Free. 

First 

29-4 

I2O 

17-4 

2OO 

12-7 

7-3 

Second 

14.9 

3-9 

no 

22.4 

13-2 

9-2 

Third 

16-4 

47 

n-7 

21-6 

13-5 

8.1 

Fourth 

17.4 

5-5 

11.9 

16.1 

4.6 

ii-5 

Fifth 

21-8 

7.6 

14-2 

i5-i 

4-2 

10-9 

Sixth 

19-6 

8-0 

ii-6 

15-9 

5-9 

100 

Mean 

19.9 

12-9 

18-5 

9-5 

In  each  field  the  upper  layer  of  soil  possesses  a 
higher  capacity  for  retaining  hygroscopic  moisture  than 
does  the  lower  layer,  but  in  field  C.  this  upper  layer  is 
thicker  than  in  field  B.  It  will  be  seen  from  the  table 
that  as  regards  total  water  there  is  no  great  difference 
between  the  two  fields,  but  when  the  hygroscopic 
moisture  is  deducted,  B.  contains  3-4  per  cent,  more 
water  available  for  the  plant.  This  difference  was 
manifested  in  the  following  season  in  the  yield,  which 
was  only  2160  Ibs.  of  grain  and  straw  on  C.  and  9200 
Ibs.  on  B. 

Though  it  is  doubtful  if  the  hygroscopic  moisture 
gathered  by  the  surface  soil  in  the  cooler  and  damper 
periods  of  the  night,  can  be  passed  on  to  the  subsoil  and 
given  up  to  the  roots,  yet  by  its  evaporation  the  next 
day  it  may  help  to  keep  the  temperature  of  the  soil 
down,  and  so  indirectly  diminish  the  loss  of  water  to  the 
soil.  Of  course  in  certain  conditions  of  air  and  soil 
temperature  there  is  condensation  upon  the  soil  of  visible 
water  which  can  be  available  to  the  crops ;  for  example, 
in  some  months  drain-gauges  yield  more  water  than  the 
rainfall,  though  a  certain  amount  of  loss  by  evaporation 
must  also  have  taken  place.  This  usually  happens  in 


88  THE  TEXTURE  OF  THE  SOIL         [CHAP.  HI. 

the  early  spring,  and  can  be  set  down  to  the  conden- 
sation of  dews  by  the  thoroughly  chilled  ground  from 
a  warm  and  moist  atmosphere.  Warington  has  sug- 
gested that  the  persistent  wetness  of  the  soil  in  February 
must  be  attributed  to  this  cause.  In  a  coarse-grained 
soil  mostly  filled  with  air,  the  cooling  of  the  surface  that 
comes  by  radiation  at  night  may  result  in  an  upward 
distillation  of  water  from  the  wetter  and  warmer  subsoil. 
Hilgard  has  suggested  this  explanation  to  account  for 
the  capacity  of  some  Californian  soils  to  maintain  a 
crop  during  a  rainless  winter,  when  the  soil  itself  shows 
only  3  per  cent  or  so  of  water. 

A.  Mitscherlich  has  made  a  number  of  determina- 
tions of  the  heat  that  is  evolved  on  moistening  dry  soil 
(benetzungs-warme),  due  to  the  condensation  of  the 
hygroscopic  water  on  the  surface  of  the  soil  particles, 
and  regards  the  figure  thus  obtained  as  of  great 
significance  in  judging  of  the  physical  properties  of  a 
soil,  since  it  provides  a  measure  of  the  total  surface  of 
all  the  particles  composing  the  soil.  He  obtained  results 
of  the  following  order,  in  calories  evolved  per  gram  of 
soil — sand  o-or,  calcium  carbonate  0-38,  sandy  soil  079, 
sandy  loam  2-37,  strong  clay  14-98,  peat  22-66.  Un- 
fortunately the  determination  is  by  no  means  an  easy 
one  to  make,  and  no  sufficient  number  of  results  have 
been  obtained  for  soils  of  known  behaviour  in  the  field 
to  enable  one  to  form  a  judgment  of  the  value  of  the 
method. 


CHAPTER   IV 

TILLAGE  AND  THE   MOVEMENTS  OF    SOIL  WATER 

Water  required  for  the  Growth  of  Crops— The  Effect  of  Drainage 
— Effects  of  Autumn  and  Spring  Cultivation,  Hoeing  and 
Mulching,  Rolling,  upon  the  Water  Content  of  the  Soil— 
The  Drying  Effect  of  Crops— Bare  Fallows— Effect  of  Dung 
on  the  Retention  of  Water  by  the  Soil. 

THE  amount  of  water  transpired  by  various  plants 
during  their  growth  has  been  investigated  by  Lawes 
and  Gilbert  at  Rothamsted,  by  Hellriegel  in  Germany, 
and  Wollny  in  Munich,  and  by  King  in  America.  The 
general  principle  upon  which  these  observers  have 
worked,  has  been  to  grow  the  plants  in  pots  and 
measure  the  amount  of  water  consumed  during  growth, 
care  being  taken  to  eliminate  or  allow  for  losses  by 
evaporation  from  the  bare  ground,  and  also  to  render 
the  conditions  of  the  plant's  life  as  similar  to  those  of 
the  open  field  as  possible.  Finally,  the  plant  is  washed 
free  of  soil,  dried,  and  weighed,  so  that  a  ratio  is 
obtained  between  the  dry  matter  produced  and  the 
water  consumed  during  growth. 

Some  of  the  numbers  obtained  are  given  in  the  table 
below :  it  will  be  seen  that  the  same  plant  gives  very 
different  results  with  the  different  observers. 

86 


90     TILLAGE— MOVEMENTS  OF  SOIL  WATER  [CHAP. 


I,  uvcs  nii'l 
Gilbert. 

Hellriegel. 

Wollny. 

King. 

Wheat  . 

225 

359 

Barley  . 

262 

310 

393 

774 

Oats 

402 

557 

665 

Red  Clover 

249 

330 

453 

Peas      . 

235 

292 

477 

447 

The  divergencies  in  these  results  are  intelligible,  if 
we  consider  that  the  "  transpiration  "  process  by  which 
the  water  is  lost,  and  the  "  assimilation "  process  by 
which  the  plant  gets  heavier,  have  no  necessary  con- 
nection, though  both  become  active  under  the  same 
stimuli  of  light  and  warmth.  Some  leaves  transpire 
rapidly  as  a  means  of  maintaining  a  low  temperature 
whilst  absorbing  large  amounts  of  radiant  energy  from 
the  sun ;  other  plants  which  have  to  resist  drought 
reduce  the  transpiration  by  a  thickened  cuticle,  or  by 
a  more  concentrated  cell  sap.  Dr  H.  Brown  has  shown 
that  of  the  radiant  energy  falling  upon  a  sunflower  leaf 
on  a  bright  August  noonday,  about  95  per  cent,  was 
consumed  in  evaporating  the  transpiration  water ;  of 
the  energy  falling  upon  the  same  leaf  in  bright  diffuse 
daylight,  only  28  per  cent,  was  used  up  in  evaporation. 
Comparing  in  these  two  cases  the  water  transpired  with 
the  carbohydrate  produced  (and  this  will  be  about  T°T  of 
the  total  dry  matter)  we  find  in  the  sunlight  the  ratio 
was  347  to  I,  in  the  diffuse  daylight  234  to  I.  Further 
investigations  are  desirable ;  but,  taking  the  whole 
group  of  observations,  we  shall  be  justified  in  assuming 
that  our  ordinary  field  crops  transpire  about  300  Ibs. 
of  water  for  each  Ib.  of  dry  matter  produced.  It  now 
remains  to  translate  this  approximate  figure  into  tons 
of  water  per  acre  required  to  grow  the  ordinary  crops. 
The  following  table  shows  the  weight  at  harvest  of  a 


IV.] 


WATER  REQUIRED  FOR  GROWTH 


fair  yield  of  the  crop  in  question,  the  percentage  of 
water  contained  in  the  crop,  the  weight  of  dry  matter 
produced  per  acre,  then  the  water  transpired  as  deduced 
from  the  dry  matter  produced,  and  in  the  last  column 
this  same  amount  of  transpired  water  recalculated  as 
inches  of  rain. 


Weight 

Per  cent. 

Weight  of 

Calculated  Water 

Crop. 

at 

of 

Dry  Matter 

transpired 

Harvest. 

Water. 

at  Harvest. 

during  Growth. 

Tons 

Tons 

Tons 

Inches  of 

per  acre. 

per  acre. 

per  acre. 

Rain. 

Wheat       . 

2-5 

18 

2-05 

615 

609 

Barley 

2 

17 

1-66 

498 

4-93 

Oats 

2-5 

16 

2'10 

630 

6*24 

Meadow  Hay 

1-5 

16 

1.26 

378 

3-74 

Clover  Hay 

20 

16 

1.68 

504 

(?) 

Swedes 

I? 

88 

204 

612 

606 

Mangolds 

30 

88 

3-6o 

1080 

10-69 

Potatoes    . 

7-5 

75 

1-87 

56i 

5-55 

Beans 

2 

17 

1-66 

498 

4-94 

It  will  be  seen  that  in  all  cases  the  amount  of  water 
transpired  by  the  crop  is  a  notable  fraction  of  the  total 
annual  rainfall,  particularly  so  in  the  case  of  a  root  crop 
like  mangolds,  which  in  the  south  and  east  of  England 
will  often  require  a  full  half  of  the  total  rain  falling 
within  the  year.  As  much  of  the  rainfall  runs  straight 
off  the  surface  into  the  ditches,  and  another  portion 
is  lost  to  the  land  by  percolation  into  the  springs,  as 
again  a  considerable  fraction  is  evaporated  at  certain 
seasons  from  the  bare  surface  of  the  soil,  it  is  evident 
that  the  water  supply,  even  in  our  humid  climate,  is 
far  from  sufficient  for  the  maximum  of  production,  and 
may  easily  fall  below  that  which  is  required  for  an 
average  crop.  Indeed,  we  may  take  it  as  a  truism  that 
the  yield  is  more  often  determined  by  the  water  avail- 
able than  by  lack  of  the  other  essentials  of  growth — 


92    TILLAGE— MOVEMENTS  OF  SOIL  WATER   [CHAP. 

light  and  heat,  manure,  etc.  Of  this  we  can  have  no 
better  proof  than  the  enormous  crops  grown  by  irriga- 
tion on  sewage  farms.  Where  the  conditions  are 
favourable,  and  the  farm  is  situated  on  a  free  draining 
sandy  or  gravelly  soil,  so  that  the  water  can  be  often 
renewed  and  drained  away  to  keep  the  soil  supplied 
with  air  as  well  as  water,  the  production  of  grass, 
cabbages,  and  other  green  crops  is  multiplied  five  or 
even  tenfold  by  the  unlimited  supply  of  water.  Speak- 
ing generally,  over  a  great  part  of  England,  where  the 
annual  rainfall  is  from  35  to  25  inches,  a  large  proportion 
of  which  falls  in  the  non-growing  season,  it  is  necessary 
to  husband  the  water  supply,  and  it  will  be  found  that 
one  at  least  of  the  objects  of  many  of  our  usual  tillage 
operations  is  the  conservation  of  the  moisture  in  the 
ground  for  the  service  of  the  crop.  From  this  point 
of  view,  the  various  operations  dealing  with  the  land 
can  now  be  considered,  such  as  drainage,  ploughingj 
hoeing,  rolling,  and  other  cultivations. 

The  Effect  of  Drainage. 

Drainage  is  usually  regarded  as  a  means  of  freeing 
the  land  from  an  excess  of  water,  but  it  also  has  an 
important  effect  in  rendering  a  higher  proportion  of  the 
annual  rainfall  available  for  the  crop,  so  that  drained 
land  will  suffer  less  from  drought  than  the  same  land 
in  an  undrained  condition. 

Land  may  require  drainage  for  various  reasons :  it 
may  possess  a  naturally  pervious  subsoil,  and  yet  be 
water-logged  owing  to  its  situation,  or  the  subsoil  may 
be  so  close  in  texture  that  percolation  is  reduced  to  a 
minimum  and  the  surface  soil  remains  for  long  periods 
almost  saturated  with  water,  especially  if  the  slope  is 
gentle  and  water  lies  after  rain  until  very  large  amounts 


iv.]  DRAINAGE  93 

soak  in.  The  flat  meadows  adjoining  a  river  are 
often  water-logged  because  their  surface  is  little  higher 
than  the  water  in  the  river  and  the  general  water  table 
in  the  adjoining  soil.  In  these  cases  tile  drains  are  of 
no  value  because  of  the  want  of  fall ;  open  cuts  and 
ditches  draw  off  the  water  best,  and  by  exposing  some 
of  the  subsoil  water  both  to  aeration  and  evaporation, 
lead  to  the  improvement  of  the, land.  Another  cause  of 
swampy  water-logged  land  is  the  rising  to  the  surface 
of  a  spring  or  a  line  of  soakage,  such  as  is  always 
formed  at  the  junction  of  a  clay  or  other  stiff  soil 
with  an  overlying  pervious  formation,  "  when  the  sand 
feeds  the  clay,"  as  the  old  rhyme  runs.  Such  wet  spots 
can  be  drained  by  tiles  or  by  an  open  ditch  cutting  the 
springs  or  the  line  of  soakage.  Land  lying  on  an 
impervious  subsoil  at  the  foot  of  a  slope  is  often  very 
wet  because  the  water  which  has  accumulated  in  the  hill 
and  soaked  downwards  is  forced  to  the  surface  by  the 
hydraulic  pressure  of  the  water  above;  such  seepage 
water  rising  to  the  surface  from  the  subsoil  is  character- 
istic of  many  valley  soils,  and  can  best  be  dealt  with  by 
a  system  of  tile  drains.  But  tile  drains  are  most 
generally  employed  and  are  of  greatest  value  in  dealing 
with  stiff  impervious  subsoils,  which  cannot  get  rid  of 
the  rain  falling  upon  them  ;  indeed,  one  of  the  prime 
improvements  effected  in  English  agriculture  was  the 
drainage  of  something  like  3,000,000  acres  of  heavy 
land  between  the  years  1840-70.  A  great  portion  of 
the  work  was  unfortunately  of  little  avail,  because  at 
first  there  was  a  tendency  to  set  the  drains  too  deep, 
at  4  feet  instead  of  the  2  to  3  feet  which  have  been 
found  to  answer  best.  The  benefits  conferred  by 
drainage  depend  upon  the  lowering  of  the  permanent 
water  table  to  the  depth  at  which  the  drains  are  laid, 
so  that  instead  of  constantly  stagnant  water  a  movement 


94    TILLAGE-MOVEMENTS  OF  SOIL  WATER   [CHAP. 

of  both  water  and  air  is  established  in  the  soil  above 
the  drain.  In  the  first  place,  the  introduction  of  the 
air  which  follows  the  water  drawn  off  by  the  drains 
brings  the  whole  depth  of  soil  into  activity,  whereas 
previously  only  the  portion  not  water-logged  was  avail- 
able. Plant  roots  cannot  grow  without  oxygen  from 
the  air,  hence  in  a  water-logged  soil  the  roots 
are  confined  to  the  surface  layer  only  ;  after  drainage 
the  roots  can  penetrate  as  far  as  the  air  extends. 
At  the  same  time,  all  the  fundamental  chemical  and 
biological  processes  of  the  soil,  such  as  nitrification  and 
weathering,  are  brought  into  action  by  the  introduction 
of  the  oxygen  upon  which  they  depend.  Later  it  will 
be  seen  that  a  water-logged  soil  results  in  the  loss  of 
nitrogen  to  the  land  when  such  manures  as  nitrate  of 
soda  are  applied  to  it.  It  is  the  extended  root  range  of 
the  crop  resulting  from  the  introduction  of  air  by 
drainage  which  enables  the  drained  land  to  resist  a 
drought  better  than  before.  In  an  undrained  soil 
the  roots  are  confined  to  a  shallow  layer,  which 
they  soon  deprive  of  all  moisture ;  further  supplies 
of  water  from  the  saturated  soil  move  upwards  very 
slowly  in  a  clay  soil,  so  that  the  plant  may  suffer 
greatly.  In  a  drained  soil,  on  the  contrary,  the  roots 
traverse  the  whole  3  feet  or  so  into  which  air  has  been 
admitted  ;  this  mass  of  soil,  even  after  it  has  given  up  by 
percolation  all  the  water  it  can,  will  still  hold  much  more 
than  is  contained  in  the  shallow  layer  alone  traversed  by 
roots  before  drainage.  Following  upon  drainage,  a  slow 
improvement  in  the  texture  of  clay  soil  is  always 
manifest :  by  the  drawing  of  air  into  the  soil,  by  the 
consequent  evaporation  and  drying,  a  certain  amount  of 
shrinkage  and  a  clotting  of  the  fine  clay  particles  result, 
which  is  never  entirely  undone  when  they  are  wetted 
again.  Roots,  which  afterwards  decay  and  leave  holes> 


IV.]  DRAINAGE  95 

and  deep  worm  tracks,  are  all  brought  into  the  soil  by  its 
aeration,  and  result  in  more  rapid  percolation.  Again, 
the  washing  through  the  soil  of  soluble  salts  derived 
from  the  surface,  especially  the  bicarbonate  of  lime 
which  is  so  characteristic  a  constituent  of  drainage 
water,  also  induces  flocculation  of  the  fine  clay  particles. 
Lastly,  there  is  a  steady  removal  by  the  drains  of  the 
finest  clay  stuff,  for  whenever  tile  drains  are  running 
freely  the  water  will  be  found  slightly  turbid  with  clay 
matter.  All  these  causes  contribute  to  establish  a  better 
texture  in  the  drained  soil,  beginning  at  the  tiles  and 
spreading  slowly  outwards.  The  other  result  of  drainage 
which  may  be  noted  here  is  the  greatly  increased  warmth 
and  earliness  of  a  drained  soil ;  the  high  specific  heat  of 
water,  and  the  cooling  produced  by  evaporation  when 
the  water  table  is  near  the  surface,  combine  to  hinder 
a  water-logged  soil  from  warming  up  under  the  sun's 
heat  in  the  spring,  so  that  undrained  land  is  notoriously 
cold  and  late. 

Effect  of  Autumn  Cultivation  upon  the  Water  Content 
of  the  Soil, 

In  regions  where  the  annual  rainfall  is  not  very  high 
and  occurs  chiefly  during  the  early  winter  months,  it  is 
important  to  get  as  much  of  it  as  possible  into  the  soil 
for  the  use  of  the  subsequent  crop.  Breaking  up  the 
stubbles  after  harvest  is  an  important  factor  in  catching 
the  winter  rain ;  all  land  which  is  to  lie  idle  through  the 
winter,  previous  to  the  sowing  of  roots  or  spring  corn 
should  be  early  turned  over  with  the  plough  and  left 
rough  through  the  rainy  season.  On  the  old  stubble 
which  has  been  made  solid  by  the  weather  and  the 
trampling  during  harvest,  the  rain  lies  for  some  time  and 
evaporates,  and  if  the  land  be  at  all  on  a  slope  the  water 
shoots  off  into  the  ditches.  But  the  broken  surface  of  a 


96    TILLAGE— MOVEMENTS  OF  SOIL  WATER  [CHAP. 

ploughed  field  both  hinders  the  flow  of  the  water  and 
affords  it  many  openings  by  which  to  sink  in ;  at  the 
same  time  the  increase  of  pore  space  in  the  loose 
ploughed  layer  enables  this  portion  to  absorb  more 
water  before  percolation  begins.  King  has  observed  in 
May  a  difference  of  2-3  per  cent  of  water  in  the  top  3 
feet  of  soil  between  land  ploughed  in  the  autumn  and 
the  adjoining  land  not  ploughed  ;  the  gain  in  this  case 
due  to  the  ploughing  was  1 10  tons  of  water  per  acre  or 
rather  more  than  I  inch  of  rain. 

The  following  table  shows  the  effect  of  ploughing  up 
a  stubble  in  autumn  on  a  thin  chalky  loam  at  Wye, 
Kent,  where  the  soil  is  only  about  2  feet  deep.  The 
samples  were  taken  on  3rd  March  1902  ;  there  had 
been  but  little  rainfall  except  in  the  previous  December. 
The  figures  show  mean  percentages  of  water  in  the  wet 
soil  exclusive  of  stones. 


Land  Ploughed 
in  Autumn. 

Adjoining  Land 
not  Ploughed. 

1st  foot 
2nd  foot 

16-45 
15.8 

16 
14-6 

Of  course  the  autumn  ploughing  has  many  other 
beneficial  effects  in  addition  to  the  above-mentioned 
gain  of  water ;  the  ploughed  soil  gets  alternately  frozen 
and  thawed,  wetted  and  dried,  with  the  result  that  on  the 
stiff  lands  the  puddling  effects  of  trampling,  etc.,  are 
obliterated,  and  the  soil  acquires  a  loose,  open  texture, 
out  of  which  a  seed  bed  can  be  made.  Again,  the 
additional  surface  which  is  exposed  to  the  action  of  frost 
and  rain  causes  increased  weathering,  and  some  of  the 
dormant  mineral  plant  food  is  brought  into  a  more 
available  condition. 


IV.]  CULTIVATION  AND  SOIL  WATER  97 

Spring  Cultivation. 

In  such  climates  as  prevail  in  parts  of  England, 
where  it  is  necessary  to  retain  as  much  of  the  winter's 
rainfall  in  the  land  as  possible,  and  where  spells  of 
drying  weather  are  apt  to  set  in  with  the  spring,  it  is 
desirable  to  cross  plough  or  otherwise  move  any  land 
that  is  destined  for  a  summer  crop  at  as  early  a  date 
as  it  will  bear  cultivation.  This  spring  working  is 
necessary  for  two  reasons :  to  obtain  a  mulch,  or  layer 
of  loose  soil,  which  will  conserve  moisture  in  the  sub- 
soil during  the  dry  periods  that  follow,  and  to  give  the 
surface  soil  an  opportunity  of  drying  gradually  into  a 
condition  that  will  yield  a  good  tilth.  The  land,  even 
though  ploughed  in  the  autumn,  will  become  consoli- 
dated again  to  a  considerable  degree  by  the  beating 
rains  of  winter.  In  this  closely  packed  material  capil- 
lary water  can  move  freely,  and  as  the  surface  layer 
dries  under  the  action  of  the  sun  and  wind,  fresh  supplies 
of  water  are  lifted  from  the  subsoil  by  surface  tension, 
with  the  result  that  there  is  a  steady  and  continuous 
drain  of  subsoil  water  through  its  connection  with  the 
exposed  and  rapidly  evaporating  surface.  But  if  the 
top  layer  of  soil  is  broken  up  and  left  loose  upon  the 
land  by  the  cultivator,  there  is  no  longer  a  continuous 
film  joining  the  exposed  surface  and  the  subsoil  water  ; 
surface  tension  can  only  lift  water  as  far  as  the  film  is 
unbroken,  t.e.,  as  far  as  the  unstirred  soil  extends,  and 
this  layer  is  protected  from  evaporation  by  the  loose 
soil  above.  Regarding  it  from  another  point  of  view — 
in  the  undisturbed  land  there  exist  fine  passages  and 
capillary  spaces  extending  from  the  surface  down  to  the 
subsoil ;  up  these  passages  water  will  rise  as  long  as  it  is 
withdrawn  by  evaporation  at  the  top ;  in  consequence, 
the  surface  soil  is  not  allowed  to  dry,  being  fed  with 

G 


98    TILLAGE— MOVEMENTS  OF  SOIL  WATER   [CHAP. 

subsoil  water  which  is  constantly  withdrawn  from  below. 
But  when  the  land  is  cultivated  the  capillary  channels 
are  broken,  water  cannot  rise  into  the  loose  layer  of 
surface  soil,  which  in  the  main  is  separated  from  the 
firm  soil  below  by  large  spaces  across  which  water 
cannot  rise ;  hence  the  surface  soil  can  become  dry, 
because  it  is  cut  off  from  the  subsoil  water,  which  in  its 
turn  is  preserved  for  use  later.  The  drying  of  the 
surface  soil  which  ensues,  through  its  severance  from 
the  water-yielding  subsoil,  is  of  the  greatest  possible 
importance  in  obtaining  a  tilth.  At  a  certain  stage 
the  soil  can  be  dragged  and  will  fall  in  pieces,  but  if 
it  be  not  detached  from  the  subsoil  it  will  either  remain 
persistently  wet,  so  that  it  cannot  be  harrowed  down, 
or  if  it  be  forced  to  dry  under  the  action  of  wind  and 
sun,  it  will  set  very  hard  and  "  steely,"  should  it  contain 
any  admixture  of  clay.  The  sudden  forced  drying  of 
strong  land  always  produces  hard  and  intractable  clods, 
which  may  defy  all  the  efforts  of  the  cultivator  during 
the  rest  of  the  season,  unless  a  fortunate  succession  of 
weather  enable  him  to  begin  to  make  his  tilth  over 
again. 

It  may  be  thought  that  the  amount  of  water  lifted 
by  surface  tension  cannot  be  so  large  as  to  result  in 
any  serious  losses  to  the  subsoil  store,  but  in  soils  of 
suitable  texture  enough  can  certainly  be  raised  to  keep 
the  crop  alive  during  periods  of  drought.  In  some  of 
King's  experiments  with  a  cylinder  full  of  very  fine 
sand,  he  found  that  the  evaporating  surface  lost  daily 
an  amount  of  water  equal  to  0-46  inch  if  the  per- 
manent water  level  were  I  foot  below,  0-405  if  the 
water  had  to  be  lifted  2  feet,  and  0-18  inch  if  the 
water  had  to  rise  4  feet  to  the  evaporating  surface. 
When  the  sand  was  replaced  by  a  clay  loam,  the  lift 
of  water  to  the  surface  was  somewhat  less,  but  in  all 


iv.]        WATER  LIFTED  BY  SURFACE  TENSION       99 

cases  the  amounts  were  probably  less  than  would  be 
realised  under  field  conditions,  because  the  evapora- 
tion was  not  enough  to  dry  the  surface,  and  was 
further  checked  by  the  formation  of  a  saline  crust  on 
the  surface.  Working  in  the  field,  King  obtained  a 
daily  loss  at  the  evaporating  surface  of  1-3  Ib.  per 
square  foot,  or  019  inch  of  water,  the  water  table 
being  from  4  to  5  feet  below  the  surface. 

The  relative  powers  of  different  soils  to  lift  water  by 
capillarity  alone  is  well  seen  during  any  long  summer 
drought,  such  as  prevailed  in  the  south  of  England 
during  1899  and  1900.  In  the  Thames  valley,  fields  of 
swedes  grew  till  the  roots  were  one  or  two  inches  in 
diameter,  and  then  died  outright,  although  the  water 
table  was  not  more  than  16  or  20  feet  below;  yet  the 
coarse-grained  gravel  of  which  the  subsoil  was  composed 
could  not  lift  the  water  in  any  appreciable  quantity  to 
the  surface.  In  the  same  seasons  the  crops  upon  the 
chalk  hills  were  quietly  growing ;  though  the  water  table 
was  as  much  as  200  feet  below  the  surface,  there  was 
still  a  steady  capillary  rise  of  water  through  the  fine- 
grained chalk.  In  a  drought  it  is  always  the  gravels 
and  coarse  sands  which  suffer  first,  and  this  not  because 
they  start  with  less  water,  for  we  have  already  seen 
that  what  they  absorb  they  can  give  up  almost  wholly 
to  the  plant,  whereas  a  clay,  which  absorbs  much  more, 
can  only  hand  over  about  the  same  proportion  to  the 
plant  as  the  sand  did,  so  much  being  held  as  hygroscopic 
moisture.  The  plant  suffers  because  the  small  surface 
of  the  soil  particles  gives  the  coarse-grained  sand  or 
gravel  a  very  limited  power  of  lifting  the  subsoil  water 
to  the  roots  of  the  plant.  Should  a  drought  continue, 
the  clay  soils  begin  to  suffer  next,  for  though  they  start 
with  large  supplies  of  water  and  have  an  extensive  sur- 
face of  soil  particles,  yet  water  can  be  moved  so  slowly 


ioo   TILLAGE— MOVEMENTS  OF  SOIL  WATER  [CHAP. 

through  the  very  fine  pore  spaces  that  the  upward  lift 
cannot  keep  pace  with  the  loss  by  transpiration  and 
evaporation.  The  soils  which  are  least  affected  by 
drought  are  the  deep  loamy  sands  of  very  uniform  tex- 
ture, fine-grained  enough  to  possess  a  considerable  lift- 
ing surface,  and  yet  not  too  fine  to  interfere  with  the 
free  movement  of  soil  water.  The  western  soils  which 
the  American  writers  describe  as  capable  of  growing 
wheat  with  a  winter  rainfall  of  10  to  12  inches  and  an  un- 
broken summer  drought  of  three  months'  duration,  are 
deep,  fine-grained,  and  uniform,  with  practically  no 
particles  of  the  clay  order  of  magnitude  to  check  the 
upward  lift  by  capillarity. 

The  following  table  illustrates  how  the  subsoil  acts 
as  a  regulator  to  the  amount  of  water  contained  in  the 
surface  layer,  absorbing  thfc  water  which  descends  by 
percolation  during  rainy  periods,  and  giving  it  up  again 
by  capillarity  to  the  surface  soil  during  periods  of 
drought  The  first  line  shows  the  rainfall  during  the 
periods  indicated,  the  second  line  the  amount  of 
evaporation  during  the  same  period,  while  the  third  line 
shows  the  changes  in  the  water  content  of  the  top  foot 
of  soil.  As  this  change  is  not  represented  by  the 
difference  between  the  rainfall  and  the  evaporation,  it 
is  clear  that  water  must  have  been  in  some  cases  passed 
down  to  the  subsoil,  in  others  lifted  from  it,  in  quantities 
shown  by  the  last  set  of  figures. 


80/iv. 

80/v. 

9/vii. 

7/iz. 

Water  in  laches. 

to 

to 

to 

to 

BO/v. 

9/vii. 

7/ix. 

27/x. 

4-i  i 

e.fic 

Evaporation  

3-45 

2-96 

5-71 

5  D5 
I-83 

Gain  or  Loss  of  Water  in  top  foot 

-  1-0 

+  1-4 

-0-24 

+  0-6  1 

Water  furnished  by  (  -  ),  or  passed 

on  to  (  +  )  Subsoil 

-2-27 

+  0-17 

-2-0 

+  3-21 

IV.] 


CULTIVATION  AND  SOIL  WATER 


101 


During  the  first  period,  the  month  of  May,  a  dry 
spell  prevailed,  only  018  inch  of  rain  fell,  while  the 
evaporation  amounted  to  3-45  inches ;  despite  this 
loss  the  top  foot  of  soil  only  contains  I  inch  of  water 
less  than  at  the  beginning,  so  that  the  rest  of  the 
excess  of  evaporation  over  rainfall  must  have  come 
from  the  subsoil,  which  had  in  fact  to  furnish  2-27 
inches.  In  the  second  period  more  water  fell  as  rain 
than  was  evaporated ;  the  surface  soil  gained  1-4  inch, 
which  did  not  account  for  all  the  excess  of  rain  over 
evaporation,  a  further -17  inch  must  have  descended 
into  the  subsoil. 

The  following  figures,  obtained  by  King,  illustrate 
how  a  spring  ploughing  preserves  the  soil  moisture 
during  a  period  of  dry  weather,  by  establishing  a 
loose  protecting  layer  ovej;  the  water  bearing  subsoil. 
The  upper  line  shows  the  water  content  of  the  top 
4  feet  of  a  certain  piece  of  land  on  2Qth  April,  on 
which  date  part  of  it  was  ploughed  and  part  left 
untouched.  On  6th  May,  no  rain  having  fallen, 
the  soil  was  sampled  again,  both  on  the  ploughed 
and  the  unploughed  piece,  with  the  results  set  out 
in  the  lower  figures  : — 


Lbs.  of  Water  in  each  successive  cubic  foot. 

1st. 

2nd. 

3rd. 

4th. 

20-1 

20-7 
18 

18 
18-3 
17-3 

16-6 
16 
13-9 

Land  on  6th  May,  ploughed  2Qth  April    . 
Land  on  6th  May,  not  ploughed 

13-9 
1  06 

It  is  seen  that  the  ploughed  land  practically  lost  no 
water  during  the  week  ending  6th  May,  whereas 
during  the  same  period  the  land  not  ploughed  lost 
9-1  Ibs  per  square  foot  of  surface,  a  quantity  equivalent 
to  if  inch  of  rain. 


loa   TILLAGE-MOVEMENTS  OF  SOIL  WATER  [CHAP. 

A  similar  trial  made  on  a  light  loam  at  Wye  during 
a  dry  period  in  the  spring  of  1902,  gave  the  following 
percentages  of  water  in  the  wet  soil. 


Land  Ploughed 
Autumn  and  Spring. 

Ploughed  Autumn 
only. 

1st  foot 
2nd  foot      . 

16-7 
15.4 

15-9 
13-9 

There  can  be  little  doubt  that  the  earlier  land 
which  is  intended  for  spring  corn,  or  particularly  for 
roots,  can  be  moved  in  the  spring,  the  more  water 
will  be  saved  for  the  use  of  the  subsequent  crop,  and 
the  easier  will  a  good  tilth  be  established.  The  chief 
danger  lies  on  the  very  fine  sandy  soils  which,  when 
in  a  loose  condition,  are  apt  to  run  together  under 
heavy  rains  and  afterwards  cake  on  drying. 

Hoeing  and  Mulches. 

The  principles  which  have  already  been  developed 
to  explain  the  effect  of  an  early  spring  ploughing  in 
saving  subsoil  water,  apply  even  more  markedly  to 
all  the  later  spring  and  summer  cultivations,  hoeing 
and  the  like,  which  have  for  their  object  the  mainten- 
ance of  a  loose  tilth  upon  the  surface.  The  loose  soil 
becomes  itself  dry,  but  by  reason  of  its  discontinuity 
and  coarse  -  grained  condition,  does  not  conduct  the 
moisture  from  the  firm  subsoil  to  the  surface  exposed 
to  sun  and  wind.  Under  these  conditions  the  only 
loss  will  be  of  that  water  which  evaporates  from  the 
moist  soil  into  the  air  spaces  of  the  loose  upper  layer 
and  then  diffuses  into  the  atmosphere ;  the  deeper  the 
loose  layer  thus  formed,  the  more  effective  will  it  be, 
and  if  it  is  destroyed  by  a  fall  of  rain,  which  consolidates 


IV.]  CULTIVATION  AND  SOIL  WATER  103 

the  ground  and  establishes  a  continuous  liquid  film 
from  the  subsoil  water  right  up  to  the  surface,  it  should 
be  renewed  by  a  fresh  cultivation  as  soon  as  the  land 
will  admit  of  working.  It  is  often  noticed  that  a  casual 
shower  during  a  dry  period,  or  watering  a  garden  unless 
the  operation  is  done  very  thoroughly,  may  result  in  a 
greater  drying  up  of  the  soil  than  ever,  just  because 
a  film  of  water  is  created  able  to  lift  water  from  the 
subsoil  up  to  the  evaporating  surface.  The  loose  hoed 
ground  practically  forms  a  mulch,  though  the  protect- 
ing material  is  the  soil  itself  instead  of  straw  or  kindred 
substances. 

Of  course,  the  conservation  of  soil  moisture  is 
not  the  only  good  effect  brought  about  by  the  surface 
cultivation  during  the  summer :  the  aeration  of  the 
soil,  the  mechanical  distribution  of  the  nitrifying 
bacteria  that  is  effected,  the  warmth  of  the  surface 
layers  due  to  their  dryness,  all  combine  to  render 
nitrification  active,  and  to  bring  into  a  form  available 
for  the  plant  the  reserves  of  nitrogen  in  the  humus  of 
the  soil.  This  point  will  be  dealt  with  more  at  length 
later :  for  the  time,  it  will  be  sufficient  to  remind  the 
reader  how  a  turnip  crop  with  its  frequent  spring  and 
summer  cultivations  is  almost  independent  of  any 
nitrogenous  manure,  though  it  removes  something  like 
100  Ibs.  of  nitrogen  per  acre  :  whereas  a  wheat  crop, 
removing  less  than  half  that  quantity  of  nitrogen  per 
acre,  often  requires  the  application  of  a  nitrogenous 
manure,  because  it  is  grown  on  undisturbed  soil  in 
the  cooler  season  of  the  year. 

The  saving  of  soil  moisture  which  can  be  effected  by 
hoeing  is  illustrated  by  one  of  King's  experiments, 
when,  during  a  dry  period,  the  soil  on  a  piece  of  land 
kept  cultivated  to  a  depth  of  3  inches  was  sampled  from 
time  to  time  down  to  a  depth  of  6  feet,  samples  being 


KM  TILLAGE— MOVEMENTS  OF  SOIL  WATER  [CHAP. 

taken  simultaneously  from  an  adjacent  piece  of  land 
where  the  surface  was  kept  smooth  and  firm.  On  the 
cultivated  land  there  was  a  daily  loss  equivalent  to 
\\\  tons  of  water  per  acre,  which  was  increased  on  the 
uncultivated  land  to  17-6  tons  per  acre;  the  difference 
during  the  49  days  over  which  the  trial  was  spread, 
amounting  to  1-7  inch  of  rain  saved  by  the  cultivation. 

The  value  of  surface  cultivation  is  well  seen  in  other 
trials  of  King's,  where  the  water  content  down  to  a 
depth  of  4  feet  was  compared  on  two  adjacent  pieces  of 
land,  one  stirred  to  the  depth  of  3  and  the  other  to  I  \ 
inches  only.  The  3-inch  soil  mulch,  taking  the  whole 
season  through,  preserved  more  soil  moisture  than  the 
shallower  cultivation,  but  by  keeping  the  soil  immedi- 
ately below  the  mulch  more  moist  and  therefore  with  a 
better  developed  water  film,  it  also  enabled  this  layer  to 
lift  more  moisture  from  the  3  or  4  foot  depth  into  the 
top  or  second  foot,  a  position  more  available  for  the 
crop.  Thus  the  average  of  three  determinations  of 
water  content  on  i6th  July  gave  the  following  results — 


Per  cent,  of  Water. 

1st  foot. 

2nd  foot. 

8rd  foot. 

4th  foot. 

Soil  cultivated,  3"  deep 
„              I  £"  deep     . 

12-3 
1  1  -2 

1  8-6 
17-6 

1  6-8 

17-8 

14-6 
l6-2 

On  this  occasion  it  is  seen  that  the  upper  2  feet  of 
soil  are  being  kept  moister  by  their  greater  power  of 
lifting  water  from  the  lower  layers,  which  actually  con- 
tain more  water  under  the  i^-inch  mulch  than  under 
the  3-inch  mulch. 

Although  the  gardener  uses  the  hoe  freely  to  estab- 
lish soil  mulches,  he  also  employs  dung,  grass-clippings, 
and  even  straw  to  the  same  end,  anything  to  break  the 


IV.]  MULCHES  105 

connection  between  the  water-bearing  subsoil  and  the 
exposed  evaporating  surface.  Such  mulches  of  loose 
organic  material  are  even  more  effective  in  conserving 
soil  moisture  than  a  fine  tilth,  there  is  less  tendency  to 
form  any  continuity  of  water  film  between  subsoil  and 
mulch ;  moreover,  the  evaporation  of  the  water  they 
themselves  contain  helps  to  keep  the  temperature  down. 
The  great  drawback  to  their  employment  is  that  they 
prevent  the  continual  stirring  of  the  ground  which 
promotes  aeration  and  nitrification. 

Stones  serve  almost  the  same  purpose  as  a  mulch, 
especially  when  they  are  impermeable,  like  flints,  and 
cover  the  surface  at  all  thickly.  They  shield  the  land 
below  from  evaporation ;  indeed,  on  picking  a  flint  off  an 
arable  field  the  ground  below  will  generally  be  found 
cool  and  damp.  The  vineyards  of  the  Rhine,  etc.,  are 
generally  set  on  steep  slopes  very  thoroughly  drained 
and  exposed  to  the  sun ;  it  will  be  noticed  that  the 
utmost  care  is  taken  to  keep  the  surface  of  the  soil 
covered  with  the  broken  slaty  rock. 

Effect  of  Rolling. 

Though  it  has  been  pointed  out  that  maintaining  a 
loose  tilth  on  the  surface  is  the  most  effective  means 
possessed  by  the  farmer  of  saving  the  soil  water  and 
minimising  losses  by  evaporation,  yet  one  of  the  funda- 
mental acts  of  husbandry  in  the  spring  consists  in  roll- 
ing and  otherwise  consolidating  the  land.  Particularly 
is  this  the  case  on  the  chalk  and  similar  light  soils  ;  when- 
ever a  spell  of  dry  weather  prevails  in  the  early  part  of 
the  year  the  farmer  will  be  observed  rolling  his  seeds, 
or  his  spring  corn,  or  his  newly  sown  turnip  land,  as 
the  case  may  be ;  he  will  even  take  a  heavy  cart 
wheel  down  between  the  drills  when  the  roller  will 
not  give  him  pressure  enough.  The  result  of  the 


106  TILLAGE— MOVEMENTS  OF  SOIL  WATER  [CHAP. 

consolidation  of  the  surface  soil  thus  effected  is  to 
improve  its  power  of  lifting  the  soil  water  from  below 
by  capillarity,  because  the  pore  space  is  diminished 
and  the  wide  intervals  across  which  the  water  film 
cannot  exist  are  largely  closed  up ;  just  as  the  motion 
of  water  through  surface  tension  almost  ceases  in  a 
thoroughly  loose  soil,  it  \s,per  contra,  increased  when 
the  particles  are  brought  more  closely  together. 
Hence,  on  the  rolled  land  there  will  be  a  greater  lift 
to  the  evaporating  surface  and  subsequent  loss  of 
water,  but  the  farmer  faces  this  loss  in  order  to  keep 
the  upper  few  inches  of  soil  supplied  with  moisture. 
Rolling  is  only  done  on  land  occupied  by  germinating 
seeds,  young  spring  corn,  or  a  young  ley,  where  the 
roots,  if  any,  are  so  close  to  the  surface  that  the  whole 
crop  will  perish  if  the  top  layer  is  allowed  to  dry.  The 
effect  of  rolling  is  to  increase  the  capillary  lifting  power 
of  the  top  soil,  so  maintaining  it  in  a  moister  condition, 
although  the  land  as  a  whole  is  made  dryer  by  the  extra 
evaporation  which  must  accompany  the  rise  of  subsoil 
water  to  the  surface.  It  is  a  maxim  in  farming  on  the 
chalk,  where  there  is  always  a  store  of  subsoil  water  at 
some  depth  or  other,  and  where  also  the  surface  soil  is 
peculiarly  liable  to  become  open  in  texture  through  the 
action  of  worms  and  the  rapid  decay  of  dung,  that  the 
land  will  become  moist  if  it  can  only  be  got  "tight" 
enough.  On  any  light  cultivated  land  it  is  easy  to 
notice  how  much  moister  the  soil  remains  when  it  has 
been  consolidated  by  a  foot  mark  ;  a  gardener  again, 
whose  rich  and  deeply- worked  soil  is  apt  to  get  very 
open,  always  treads  the  ground  as  solid  as  possible  in 
preparing  a  seed  bed  for  onions  and  other  small  seeds. 
The  following  figures  given  by  King  as  mean  values 
from  a  number  of  measurements  show  how  rolling  dries 
the  soil  as  a  whole  when  samples  are  taken  down  to  2 


iv.]  ROLLING  107 

feet  or  more,  but  maintains  the  surface  soil,  sampled 
only  down  to  18  inches,  in  a  moister  condition. 


Depth  of  Sample. 

PERCENTAGE  or  WATER. 

Rolled  Ground. 

Unrolled  Ground. 

Down  to  1  8"   . 
Down  to  24"    . 
Down  to  36"-  54" 

I5-85 
19-49 
18-72 

I5-64 
19-85 

19-43 

Since  rolling  dries  the  soil  as  a  whole,  it  is  only  desirable 
when  shallow-rooted  crops  must  be  kept  supplied  with 
water  at  any  cost ;  as  soon  as  they  get  their  roots  down 
hoeing  should  begin  to  diminish  the  inevitable  evapora- 
tion from  the  firm  surface.  Thus  a  tool  like  the  old 
broadsharing  plough,  still  used  on  the  chalk,  is  particu- 
larly valuable  in  preparing  a  tilth  for  roots,  for,  while 
creating  a  loose  surface  tilth,  it  is  consolidating  the  soil 
below  and  increasing  its  power  of  lifting  water  from  the 
subsoil. 

Similarly,  in  the  semi-arid  regions  of  Western 
America  and  in  Australia,  where  the  rainfall  is  barely 
sufficient  for  the  needs  of  the  crop,  in  the  preparation  of 
the  land  great  importance  is  laid  on  the  two  operations 
of  "subsoil  packing,"  and  "the  establishment  of  a  soil 
mulch."  This  is  the  equivalent  of  the  English  practice 
of  preparing  a  seed  bed  for  roots ;  frequent  cultivation 
without  inverting  the  soil  to  work  it  down  to  a  fine 
tilth,  constant  use  of  the  ring  roller  or  subsoil  packer  to 
consolidate  this  crumb  until  it  will  lift  water  by  capillarity, 
and  finally  the  production  and  continual  renewal  by 
means  of  light  cultivators  or  horse  hoes  of  a  very  thin 
skin  of  loose  soil  on  the  surface. 

Valuable  as  the  operation  of  rolling  is  on  grass 
land  in  the  early  spring,  in  order  to  consolidate  the  soil 


io8  TILLAGE— MOVEMENTS  OF  SOIL  WATER   [CHAP. 

round  the  roots  of  the  grass  after  the  surface  has  been 
lifted  by  the  winter  frosts  and  by  the  action  of  worms, 
it  should  be  borne  in  mind  that  it  is  easy  to  do  harm 
by  injudicious  rolling  in  wet  weather  on  soils  that  are 
at  all  heavy.  Even  on  grass  land  the  clay  may  become 
so  puddled  or  tempered  that  it  dries  round  the  roots 
with  a  very  harsh  caked  surface,  little  permeable  to 
air  and  water.  This  sort  of  damage  is  perhaps  most 
often  seen  on  lawns  and  cricket  grounds  which  are 
often  rolled  repeatedly  with  heavy  rollers  when  the 
ground  is  thoroughly  wet ;  a  smooth,  pasty  surface  is 
produced  to  the  ultimate  great  detriment  of  the  growth 
of  the  grass.  Of  course  upon  arable  land  the  greatest 
care  must  be  taken  never  to  roll  when  the  top  is  at  all 
wet  or  even  damp,  lest  a  pasty  surface  be  developed, 
which  will  dry  to  a  glazed  baked  crust  It  is  necessary 
even  to  wait  until  the  dew  has  been  dissipated  before 
rolling  strong  land  that  has  been  well  worked  and 
drilled  for  roots. 

The  Drying  Effect  of  Crops. 

Since  a  crop  transpires  about  300  Ibs.  of  water  for 
each  pound  of  dry  matter  produced,  any  land  which 
is  carrying  a  heavy  crop  must  contain  much  less  water 
than  the  adjoining  uncropped  land,  unless  there  has 
been  such  an  excess  of  rainfall  as  to  saturate  the  soil 
in  either  case.  Any  summer  growing  crop,  however, 
especially  one  of  roots,  transpires  so  large  a  proportion 
of  the  customary  rainfall  during  the  period  of  growth, 
that  it  must  leave  the  soil  much  drier  for  its  growth. 
As  an  example  of  this  removal  of  water  by  the  growing 
crop,  the  following  figures  obtained  at  Rothamsted 
during  the  very  dry  summer  of  1870  may  be  quoted, 
showing  as  they  do  the  water  present  in  successive  9 


IV.]  WATER  EVAPORATED  BY  CROPS  109 

inches  of  fallow  and  of  adjoining  land  carrying  a  barley 
crop — 


Percentages  of  Water  in  fine  Soil, 
June  27-28,  1870. 

Fallow. 

Barley. 

First  9" 

20-36 

1  1  -91 

Second  9" 

29-53 

19-32 

Third  9 

34-84 

22-83 

Fourth  9" 

34-32 

2509 

Fifth  9" 

31-31 

26-98 

Sixth  9" 

33-55 

26-38 

The  total  difference  between  the  cropped  and  un- 
cropped  land  down  to  the  depth  of  54  inches,  amounted 
to  more  than  900  tons  of  water  per  acre,  or  9  inches 
of  rain,  which  is  quite  half  as  much  again  as  would 
be  accounted  for  by  the  crop  on  the  assumption  that 
only  two  tons  or  so  of  dry  matter  had  been  grown  at 
the  date  of  sampling. 

Another  example  of  the  withdrawal  of  water  from 
the  soil  by  the  crop  is  seen  in  the  proportions  of  water 
in  the  soil  of  certain  of  the  permanent  grass  plots  at 
Rothamsted,  taken  in  July  of  the  same  year,  1870 — 


PLOT  3. 

PLOT  9. 

PLOT  14. 

Mineral  Manure 

Mineral  Manure 

No  Manure. 

.(- 

_(- 

Ammonium  Salts. 

Nitrate  of  Soda. 

Crop,  1870  . 

5  j  cwt.  of  Hay. 

29J 

56J 

Per  cent,  of  Water. 

First  9" 

10-83 

I3-OO 

12-16 

Second  9" 

13-34 

10-18 

1  1  -80 

Third  9" 

19-23 

16-46 

I5-65 

Fourth  9" 

22-71 

18-96 

16-30 

Fifth  9" 

24-28 

20-54 

17-18 

Sixth  9" 

25-07 

21-34 

1  8-06 

Means  . 

19-24 

16-75 

15-19 

no  TILLAGE— MOVEMENTS  OF  SOIL  W 'ATE R  [CHAP. 

Down  to  the  depth  of  54  inches  the  plot  receiving 
minerals  and  ammonium  salts  contained  200  tons,  and  the 
plot  receiving  minerals  and  nitrate  325  tons,  less  water 
than  the  unmanured  plot,  quantities  in  this  case  some- 
what less  than  would  be  indicated  by  the  amount  of 
dry  matter  produced. 

There  are  two  important  cases  in  which  the  drying 
effect  of  vegetation  needs  to  be  taken  into  account,  in 
the  use  of  catch  crops  and  in  the  planting  of  fruit  trees. 
On  the  lighter  lands  of  the  south  of  England  catch 
crops  are  not  uncommonly  taken  on  the  land  before 
roots.  The  stubbles  are  quickly  broken  up,  and 
vetches,  trifolium,  or  rye,  are  sown  in  time  to  make  a 
start  while  the  land  is  warm,  and  to  be  either  cut  green 
or  fed  off  before  the  land  is  wanted  for  turnips  in  the 
following  spring.  The  advantages  of  the  practice  are 
that  the  summer-formed  nitrates  in  the  stubble-ground 
are  saved  from  washing  out,  and  that  a  valuable  bite  of 
early  fodder  is  obtained  :  with  the  leguminous  crops  also, 
the  farm  is  enriched  by  the  nitrogen  gathered  from  the 
atmosphere.  The  difficulty  of  getting  catch  crops  lies 
in  the  fact  that  the  stubble  ground  is  left  very  dry  by 
the  preceding  crop,  so  that  a  timely  rainfall  is  needed  to 
obtain  a  plant.  The  danger  of  their  use  is  that  they 
may  so  deplete  the  available  soil  water  as  to  give  the 
succeeding  crop  of  roots  a  very  poor  chance  of  germin- 
ating or  growing  well.  In  America  the  practice  has  been 
suggested  of  sowing  some  leguminous  crop  like  clover 
in  the  tillage  orchards  about  the  end  of  July,  so  that  the 
new  surface  crop  should  so  dry  the  ground  as  to  forward 
the  ripening  of  the  apples  on  the  trees ;  again,  any 
second  growth  of  the  trees  due  to  a  late  summer  rainfall 
would  be  prevented,  this  moisture  being  dealt  with  by 
the  catch  crop. 

The  second  illustration  worthy  of  notice  is  that  fruit 


iv.]    PLANTING  FRUIT  TREES  IN  GRASS  LAND    in 

trees  when  newly  planted  in  grass  land  often  make  a 
very  poor  growth  for  a  year  or  two.  This  is  because  a 
fruit  tree  when  planted  is  but  indifferently  supplied  with 
water-collecting  roots  ;  inevitably  they  are  few  in  number 
and  have  a  very  restricted  range.  Hence  they  must  be 
in  a  soil  well  supplied  with  moisture  if  they  are  to  provide 
the  tree  with  the  necessary  water,  and  they  are  very  ill 
fitted  to  compete  with  a  crowd  of  fibrous  grass  roots 
surrounding  them,  should  the  season  turn  out  dry.  In 
one  experiment  the  moisture  in  the  top  foot  of  a 
pasture  was  found  to  be  only  half  that  present  in  the  top 
foot  of  neighbouring  uncropped  land. 

The  following  table  shows  the  percentages  of  water 
in  the  fine  earth  of  an  orchard  on  heavy  soil,  part  of 
which  was  under  grass  and  part  kept  tilled ;  it  will  be 
seen  that  in  the  winter  the  grass  land  carries  as  much 
or  even  more  water  than  the  bare  soil,  but  towards  the 
end  of  the  summer  the  drying  effect  of  the  grass  becomes 
very  pronounced,  even  down  to  the  third  foot. 


IST  9  INCHES. 

2ND  9  INCHES. 

SRD  9  INCHES. 

Grass. 

Bare. 

Grass. 

Bare. 

Grass. 

Bare. 

December   19/05 

25-0 

24-7 

23-5 

23-5 

250 

27-0 

March  3/06 

26-7 

23-3 

21-2 

21-9 

20-6 

19-6 

May  24/06 

17-7 

24-0 

18-7 

24-5 

22-3 

250 

July  25/06 

13-8 

I5.6 

14-5 

18-2 

14-8 

24-4 

September  27/06 

12-6 

15-7 

13-8 

15.1 

15.6 

18-8 

April  6/07  . 

2I-I 

21-6 

2I-O 

240 

19-6 

21-3 

October  9/07      . 

22-6 

27.1 

237 

23-6 

250 

Few  crops  so  effectually  dry  the  surface  soil  as  grass 
does,  because  of  the  intimate  way  in  which  its  roots 
traverse  the  soil ;  hence  a  fruit  tree  cannot  compete 
with  grass  for  water  as  long  as  the  two  sets  of  roots  are 
confined  to  the  same  layer.  The  experiments  at  the 


ii2   TILLAGE— MOVEMENTS  OF  SOIL  WATER  [CHAP. 

Woburn  Fruit  Farm  of  planting  fruit  trees  and  sowing 
the  seed  of  coarse  meadow  grasses  at  the  same  time, 
show  this  competition  at  its  highest  degree,  but  even 
when  trees  are  planted  in  old  pasture  care  should  be 
taken  to  keep  a  ring  round  the  tree  free  from  grass 
and  well  cultivated  or  mulched  for  at  least  two  years. 
For  similar  reasons,  when  trees  are  planted  in  arable 
land  weeds  should  be  kept  down,  nor  should  crops  like 
cabbages  or  mangolds  be  grown  between  the  rows 
of  trees ;  such  crops  are  usually  considered  to  "  draw 
the  land "  and  deplete  it  of  plant  food,  but  the  harm 
they  do  lies  in  the  water  they  withdraw  just  at  the  most 
critical  season,  when  the  tree  is  making  its  first  start  in 
its  new  quarters. 

Bare  Fallows. 

The  custom  of  fallowing  land,  of  leaving  it  entirely 
bare  for  a  season,  during  which  the  land  is  worked  as 
often  as  possible,  is  one  of  the  oldest  in  agriculture ;  a 
rotation  of  wheat,  wheat,  fallow,  or  of  beans,  wheat, 
fallow,  being  almost  universal,  until  the  introduction  of 
turnips  gave  the  farmer  a  chance  of  cleaning  his  land 
and  yet  growing  a  crop  at  the  same  time.  The  objects 
of  a  fallow  were  various  :  in  the  first  place,  the  summer 
cultivations  resulted  in  a  thorough  cleaning  of  the  land 
and  in  a  free  development  of  nitrates  for  the  succeeding 
crop ;  also  on  the  heavy  soils,  which  are  the  most  suited 
to  fallowing,  a  good  tilth  was  obtained  that  was  often 
impossible  otherwise.  Indeed,  at  the  present  day  it  is 
found  desirable  and  even  necessary  to  introduce  an 
occasional  bare  fallow  when  farming  on  the  heavy  clays 
of  the  south  and  east  of  England,  in  order  to  obtain  a 
satisfactory  tilth  in  that  dry  climate. 

One  of  the  most  notable  effects  of  fallowing  lies  in 
the  production  of  a  stock  of  nitrates  from  the  stores  of 


iv.]     STORAGE  OF  WATER  BY  BARE  FALLOWS    113 

combined  nitrogen  in  the  soil ;  these  nitrates  are  at  once 
available  for  the  ensuing  wheat  crop  if  the  autumnal 
rains  are  not  too  great  to  wash  them  out  of  the  soil 
(see  p.  1 1 6). 

But  in  addition  to  the  gain  in  available  nitrogen  due 
to  fallowing,  the  land  which  does  not  carry  a  crop  during 
a  season  will  accumulate  a  store  of  water  which  may  be 
of  the  utmost  service  to  the  succeeding  crop.  In  the 
preceding  section  some  figures  have  been  given  showing 
how  much  more  water  is  present  at  the  end  of  the 
summer  in  the  fallow  land  than  in  the  land  which  had 
carried  a  crop,  so  that  in  districts  where  the  winter  rain- 
fall is  small  the  fallowed  land  will  start  the  next  season 
with  a  great  advantage.  Indeed,  in  a  semi-arid  climate 
where  the  annual  rainfall  is  insufficient,  satisfactory  crops 
may  yet  be  grown  in  alternate  years  by  using  an  inter- 
mediate fallow  period  in  which  to  accumulate  a  reserve 
of  subsoil  water. 

The  following  series  of  measurements  will  illustrate 
this  point ;  it  shows  the  percentages  of  water  in  spring 
and  autumn  on  fallow  and  cropped  land  respectively, 
also  the  water  present  in  the  same  land  in  the  following 
spring  and  autumn,  when  both  plots  were  in  oats. 


SPRING. 

AUTUMN. 

FOLLOWING 
SPRING  AND  AUTUMN. 

Fallow. 

Corn. 

Fallow. 

Corn. 

Oats. 

Oats. 

Oats. 

Oats. 

1. 

2. 

1. 

2. 

l. 

2. 

1. 

2. 

1st  foot 

24% 

22% 

17% 

7% 

19% 

16% 

6% 

4% 

2nd  foot    . 

20  „ 

19   ,, 

20   „ 

12    „ 

21    „ 

18  „ 

10    ,, 

5  ,, 

3rd  foot     . 

18  „ 

18  „ 

16  „ 

4  ,, 

18  „ 

15  „ 

9  » 

8  „ 

The  effect  of  the  fallowing  in  retaining  more  moisture 
in  the  soil  is  seen  throughout  the  whole  of  the  second 
season. 

H 


114   TILLAGE— MOVEMENTS  OF  SOIL  WATER   [CHAP. 

At  Rothamsted  portions  of  the  wheat  field  were 
fallowed  during  the  summer  of  1904,  and  the  following 
table  shows  the  percentage  of  water  in  the  fine  earth  on 
1 3th  September,  2-849  inches  of  rain  having  fallen  since 
the  crops  had  been  cut. 


UNMANORED. 

DUNOKD. 

MEAN  OF  8  PLOTS. 

Cropped. 

Fallow. 

Cropped. 

Fallow. 

Cropped. 

Fallow. 

1st  9"      . 
2nd  9"    . 
3rd  9"     . 
4th  9"     . 

15-8 
18-9 
20-8 

23-1 

16-0 
19-8 

23-3 
25-2 

20-2 
14-5 
13-7 

15-5 

19-3 
I7-O 
1  8-4 
19.7 

17-4 
18-8 
2O-  1 
20-9 

17-2 
200 
22-3 
23-1 

Mean 

19-6 

20-8 

160 

18-6 

19-3 

2O-6 

In  the  surface  layer  there  is  practically  no  difference, 
both  having  become  equally  wet  by  the  rains  after 
harvest,  but  in  the  lower  depths  the  fallow  soils  are  the 
wetter,  and  the  differences  are  more  pronounced  for  the 
unmanured  plot  where  a  small  crop  had  been  grown 
than  for  the  dunged  plot  with  its  larger  crop. 

The  way  in  which  fallowed  land  is  of  benefit  to 
the  crop,  both  by  making  nitrates  and  particularly  by 
saving  water  in  a  dry  season,  is  easily  seen  in  the 
superior  plant  always  found  on  the  outside  rows  or 
edges  of  an  experimental  plot  divided  from  the  others 
by  a  bare  path ;  on  one  side  the  plant  has  the  benefit 
of  fallow  ground  as  well  as  of  extra  space,  light,  and 
air,  and  flourishes  accordingly.  The  Lois-Weedon 
system  of  husbandry,  where  the  land  was  divided 
into  alternate  5-foot  strips  of  corn  and  cultivated 
fallow  land,  was  nothing  but  an  application  of  this 
principle  on  a  large  scale,  as  indeed  is  any  system  of 
growing  a  crop  in  wide  rows  to  admit  of  some  form 
of  hoe  or  cultivator  working  regularly  at  the  ground 


IV.] 


VALUE  OF  BARE  FALLOWS 


between.  In  a  humid  climate  or  on  a  porous  soil  there 
is  great  danger  of  losing  the  nitrates  formed  in  the 
summer  by  washing  out  during  the  autumnal  and  winter 
rains,  nor  is  there  any  advantage  gained  by  storing  water 
where  the  usual  winter  rainfall  is  sufficient  to  saturate 
the  soil.  For  this  reason,  in  the  Rothamsted  experi- 
ments, the  plot  growing  wheat  continuously  has  given  a 
greater  crop  per  acre  per  annum  than  the  plot  fallowed 
and  sown  with  wheat  in  alternate  years,  though  the 
wheat  crop  following  fallow  has  always  been  larger  than 
the  crop  grown  the  same  year  on  the  unmanured  plot 


WHEAT  EVERY  YEAR. 

FALLOW  AND  WHEAT. 

Grain. 

Straw. 

Grain. 

Straw. 

Bushels. 

Lbs. 

Bnshels. 

Lbs. 

1856-1895     . 

I2| 

• 

1127 

,,-#"- 

798 

Of  course  the  average  crop  on  the  fallowed  ground 
was  twice  the  above  figures,  i.e.,  i/f  bushels  of  grain  and 
1 595  Ibs.  of  straw,  but  it  was  only  grown  every  alternate 
year. 

That  the  autumnal  rainfall  is  the  great  factor  in 
determining  whether  a  bare  fallow  shall  be  profitable 
or  not  to  the  following  crop,  may  be  well  seen  by  a 
further  examination  of  the  results  obtained  at  Rotham- 
sted on  these  plots,  by  comparing  the  crops  with  the 
percolation  which  took  place  in  the  autumn  previous. 

The  percolation  through  60  inches  of  bare  soil  for 
the  four  months,  September  to  December  inclusive,  as 
measured  by  the  drain  gauge  previously  described  on 
p.  78,  amounted  on  the  average  to  6-45  inches  for  the 
31  seasons  1870-1901.  If,  then,  we  divide  the  harvest 
years  into  two  groups  according  as  the  autumnal 
percolation  is  above  or  below  the  average,  and  allot 


ii6    TILLAGE—MOVEMENTS  OF  SOIL  WATER   [CHAP. 

to  each  year  the  crops  on  the  continuous  wheat  and 
wheat  after  fallow  plots  for  the  harvest  following  the 
given  percolation,  we  shall  obtain  the  following 
average  results,  which  show  in  group  I  the  mean 
crops  following  autumns  of  less  than  average  percola- 
tion, and  in  group  2  those  following  autumns  of 
comparatively  high  percolation.  The  percolation  is 
given  in  inches,  the  crops  in  Ibs.  per  acre  of  total 
produce,  both  grain  and  straw  ;  and  as  further  evidence 
of  the  extent  of  percolation,  the  average  number  of 
days  are  given  during  the  four  months  on  which 
water  ran  from  the  tile  drain  underlying  the  con- 
tinuous wheat  plot 


£ 

60 

a 

CROP,   LBS.    PKB  ACRE. 

a   . 

"3  • 

. 

II 

i 

g| 

°.S 

I*         . 

{»• 

5  . 

£  £ 

i 

"£  § 

•3"° 

•**  JO* 

-»^ 

•5.2 

"°S 

^53 

s£-g 

*2 

£r* 

41 

§ 

£ 

0 
S5 

^ 

^ 

O 

I. 

1  5  Years  of  Percolation 

below  average  . 

8-94 

3-99 

4 

1807 

2677 

870 

2. 

1  6  Years  of  Percolation 

above  average  . 

13-78 

8-92 

13 

1627 

1757 

130 

Thus  the  bare  fallow  which  increased  the  succeeding 
crop  above  that  given  by  the  continuous  wheat  plot 
by  nearly  48  per  cent  in  the  seasons  when  a  com- 
paratively dry  autumn  succeeded  the  fallow,  increased 
it  by  less  than  8  per  cent,  when  there  was  much 
percolation  after  the  fallow. 

It  follows,  therefore,  that  the  practice  of  fallowing 
land  is  only  an  economical  one  where  the  annual  rain- 
fall is  low  and  where  the  land  is  too  strong  to  admit 
of  free  percolation  ;  it  is,  however,  admirably  adapted  to 
the  successful  cultivation  of  clay  land  in  dry,  hot  climates. 


IV.] 


HUMUS  AND  SOIL  WATER 


117 


Effect  of  Dung  on  the  retention  of  Water  by  the  Soil. 

A  soil  which  has  been  enriched  in  humus  through 
repeated  applications  of  farmyard  manure  will  resist 
drought  better  than  one  in  which  the  humus  is  low  ;  the 
difference  is  seen  not  so  much  in  the  greater  amount 
of  moisture  present  in  the  soil  containing  humus,  as 
in  the  way  it  will  absorb  a  large  amount  of  water 
temporarily  during  heavy  rainfall,  and  then  let  it 
work  more  slowly  down  into  the  soil,  thus  keeping 
it  longer  within  reach  of  the  crop.  Good  examples 
are  afforded  by  the  Rothamsted  plots ;  samples  of 
soil  from  the  wheat  land  were  taken  on  I3th  September 
1904,  on  the  previous  day  0-262  inch  of  rain  had 
fallen,  but  for  nine  days  before  there  had  been  little 
or  no  rain.  The  portions  of  the  plots  from  which 
the  samples  were  drawn  had  been  fallowed  through 
the  summer,  so  that  the  drying  effect  of  the  crop 
is  eliminated.  Samples  were  also  taken  from  the 
barley  plots  on  3rd  October  of  the  same  year ;  0-456 
inch  of  rain  had  fallen  on  the  3Oth  September,  before 
which  there  had  been  fifteen  days  of  fine  weather. 
The  following  table  shows  the  water  in  the  soil  of  the 
unmanured  and  the  continuously  dunged  plots  respec- 
tively, as  percentages  of  the  fine  earth  from  which  the 
stones  had  been  sifted. 

Percentages  of  Water  in  Rothamsted  Soils. 


Deptb. 

BROADBALK  WHBAT. 

Hoos  BARLEY. 

Unmanured. 

Dunged. 

Unmanured. 

Dunged. 

o"  to     9" 

9"  „    I  8" 
18"  „   27" 

16-0 
19-8 
23-3 

19-3 
I7-O 
18-4 

17-0 
32-5 
22-1 

20-7 

17-7 
18-3 

n 8   TILLAGE-MOVEMENTS  OF  SOIL  WATER  [CHAP. 

It  is  thus  seen  that  in  both  cases  the  dunged  soil, 
rich  in  humus,  had  retained  more  of  the  comparatively 
recent  rainfall  near  the  surface,  so  that  the  top  soil  was 
moister  while  the  subsoil  was  drier.  The  difference 
in  favour  of  the  surface  soil  was  about  3-5  per  cent.,  which 
on  that  soil  would  amount  to  about  30  tons  per  acre, 
or  approximately  0-3  inch  of  rain.  It  is  thus  seen 
that  the  surface  soil  of  the  dunged  plot  had  retained 
practically  the  whole  of  the  preceding  rainfall ;  and 
the  greater  dryness  of  the  subsoil  was  due  to  the  way  the 
soil  had  kept  back  the  small  rainfalls,  which  have 
been  evaporated  instead  of  passed  on  to  the  subsoil 
as  they  were  on  the  unmanured  plots.  The  same  fact 
is  illustrated  by  the  behaviour  of  the  drains  which  run 
below  the  centre  of  each  of  the  wheat  plots  at  a  depth 
of  30  inches ;  below  the  dunged  plot  the  drain  very 
rarely  runs,  only  after  an  exceptionally  heavy  and 
long  -  continued  fall,  whereas  the  drain  below  the 
unmanured  plot  runs  two  or  three  times  every  winter. 
Putting  aside  the  greater  drying  effect  of  the  much 
larger  crop  on  the  dunged  plot,  the  difference  is  mainly 
due  to  the  way  the  surface  soil  rich  in  humus  first 
of  all  absorbs  more  of  the  water,  and  then  lets  the 
excess  percolate  so  much  more  slowly  that  the  descend- 
ing layer  of  over-saturation,  which  causes  the  drain  to 
run,  rarely  or  never  forms. 

The  water-retaining  power  of  the  dung  may  also 
be  seen  in  the  superior  yield  of  the  dunged  plots 
in  markedly  dry  seasons.  The  following  table  shows  a 
comparison  of  the  yield  on  plot  2,  receiving  14  tons 
of  dung,  and  plot  7,  receiving  a  complete  artificial 
manure,  for  the  years  1879,  which  was  exceptionally 
wet  and  cold,  and  1893,  which  was  hot  and  dry 
throughout  the  growing  period  of  the  plant  The 
rainfall  for  this  period,  i.e.,  for  the  four  months  March 


IV.] 


HUMUS  IN  WET  AND  DRY  SEASONS 


119 


to  June,  was  13  inches  in   1879  and   only  2-9   inches 
in  1893. 

WHEAT.    YIELD  IN  BUSHELS  OF  GRAIN. 


Plot. 

1879. 

189S. 

Avenge 
51  Yean. 

2 

16-0 

34-25 

35-7 

7 

16-25 

20-25 

32-9 

The  average  yield  on  the  dunged  plot  is  about  3 
bushels  more  than  on  plot  7,  but  in  the  dry  year  its 
superiority  amounted  to  14  bushels,  whereas  in  the 
very  wet  year  the  two  plots  sank  to  the  same  low  level. 
In  a  bad  season  the  bacterial  changes  which  render  the 
plant  food  in  dung  available  for  the  crop  go  on  very 
slowly. 


CHAPTER  V 

THE  TEMPERATURE  OF  THE  SOIL 

Causes  affecting  the  Temperature  of  the  Soil — Variation  of 
Temperature  with  Depth,  Season,  etc.  —  Temperatures 
required  for  Growth — Radiation — Effect  of  Colour — Specific 
Heat  of  Soils — Heat  required  for  Evaporation — Effect  of 
Situation  and  Exposure — Early  and  Late  Soils. 

THE  life  of  a  plant  is  practically  suspended  below 
a  certain  temperature,  which  is  about  41°  F.  for  the 
majority  of  cultivated  plants ;  all  the  various  changes 
which  are  essential  to  the  development  of  the  plant 
such  as  germination,  vegetative  activity,  and  the  bac- 
terial processes  in  the  soil,  show  a  similar  dependence 
upon  temperature. 

These  vital  actions  cease  below  a  certain  minimum, 
above  which  they  usually  increase  with  the  tempera- 
ture until  an  optimum  is  reached,  when  the  action  is 
at  its  greatest ;  beyond  this  point  the  action  decreases 
until  a  superior  limit  is  reached,  which  again  suspends 
all  change.  It  therefore  becomes  important  to  study 
the  manner  in  which  heat  enters  and  leaves  the  soil, 
because  upon  the  temperature  acquired  depend  such 
practical  questions  as  the  suitability  or  otherwise  of 
the  land  for  particular  crops,  the  season  at  which  to 
sow,  and  the  earliness  or  lateness  of  the  harvest 

The  surface  soil  receives  heat  in  four  ways  : — 
(i)  By  direct  radiation  from  the  sun,  whose   rays 

both  of  light  and  invisible  heat  are  absorbed 
120 


CHAP,  v.]  AGENCIES  AFFECTING  TEMPERATURE  121 

and  raise  the  temperature  of  the  absorbing 
soil. 

(2)  By  precipitation,  as  in  the  spring  when  warm 

rain  enters  the  ground  and  brings  with  it  a 
considerable  quantity  of  heat,  or  when  aqueous 
vapour  in  the  air  is  condensed  on  the  colder 
soil. 

(3)  By  conduction  from  the  heated  interior  of  the 

earth  a  small  amount  of  heat  reaches  the 
surface. 

(4)  By  the  changes  which  result  in  the  decay  of  the 

organic  material  of  the  soil,  when  as  much  heat 
is  developed  as  if  the  same  material  had  been 
burnt  in  a  fire. 

The  surface  soil  loses  heat : — 

(1)  By  radiation;  like  any  other  body   possessing 

heat,  the  surface  of  the  soil  is  always  emitting 
invisible  radiant  heat,  which  may,  or  may 
not,  be  counterbalanced  by  the  corresponding 
radiations  it  is  absorbing. 

(2)  By  conduction  either  to  cooler  layers  of  earth 

below  or  to  cooler  air  above. 

(3)  By  the  evaporation  of  the  water  contained    in 

the  soil ;  at  ordinary  temperatures  the  evapora- 
tion of  i  Ib.  of  water  would  absorb  enough 
heat  to  lower  the  temperature  of  about  7500  Ib. 
of  soil  by  i  °  F. 

The  actual  temperature  attained  by  a  given  soil  at 
any  time  depends  upon  the  relative  effect  of  the  heat- 
ing and  cooling  actions  set  out  above. 


122  THE  TEMPERATURE  OF  THE  SOIL       [CHAP. 


Soil  Temperatures. 

The  accompanying  curves  (Fig.  8),  show  the 
monthly  mean  temperatures  of  the  soil  at  6  inches,  3 
feet,  and  6  feet  respectively,  as  compiled  from  readings 
taken  at  9  A.M.  at  Wye  during  1896,  the  soil  being  a 
light  well-drained  loam  under  grass.  It  will  be  seen 
that  the  variations  in  temperature  diminish  with  the 
depth :  in  fact  a  point  is  soon  reached,  about  50  feet 
down,  below  which  the  effect  of  the  gain  or  loss  of 
heat  at  the  surface  is  inappreciable,  and  the  tempera- 
ture is  constant  from  day  to  day,  only  increasing  with 
the  depth,  according  to  the  well-known  law.  Each 
curve  cuts  each  other  curve  at  least  twice ;  for  a 
certain  period  the  upper  layer  is  giving,  and  during 
the  rest  of  the  year,  receiving  heat  from  the  layer 
above  or  below.  The  maximum  temperature  attained 
at  a  depth  of  3  feet  comes  a  little  later  in  the  year 
than  the  maximum  for  3  inches,  and  the  maximum 
at  6  feet  lags  still  further  behind,  owing  to  the  slow- 
ness with  which  the  heat  is  conducted.  It  will  be 
seen  that  the  curve  indicating  the  temperature  at  6 
inches  (and  the  mean  figures  for  3  and  9  inches  are 
almost  identical)  does  not  reach  the  41°  F.  required  for 
the  beginning  of  vegetative  growth  until  April ;  it 
is,  however,  constructed  from  monthly  averages  only, 
and  from  observations  taken  at  9  A.M.,  when  the 
surface  soil  has  been  considerably  cooled  during  the 
night.  Much  higher  temperatures  are  obtained  during 
certain  parts  of  the  day  even  in  the  early  spring 
months,  otherwise  no  germination  could  take  place; 
these  diurnal  and  hourly  fluctuations  are,  however, 
chiefly  confined  to  the  surface  soil.  The  following 


) 


s 

v 
H 

'S 

f 


[7o 


V-] 


VARIATIONS  OF  TEMPERATURE 


123 


curves  show  firstly  (Fig.  9),  the  daily  results  during 
a  fortnight  of  April  1902,  also  (Fig.  10)  certain  hourly 
readings  obtained  in  the  same  month,  in  this  case 
beneath  smooth,  well-worked  arable  land.  The  diurnal 
variations  die  away  before  the  depth  of  3  feet  is  reached, 
nor  are  hourly  variations  perceptible  at  the  depth  of 
one  foot,  except  in  the  case  of  heavy  precipitation  and 
a  pervious  soil.  It  will  also  be  noticed  from  the 
last  curves  that  during  part  of  the  day  the  temperature 
at  the  depth  of  6  inches  ran  up  to  a  point  well  above 
the  minimum  required  for  germination,  although  the 
mean  soil  temperature  at  9  A.M.  was  near  that  limit. 


Temperatures  required  for  Growth. 

Reference  has  already  been  made  to  the  fact  that  a 
certain  temperature  is  necessary  before  the  vital 
processes  involved  in  growth  become  active;  this 
temperature  is  not  always  the  same,  but  may  be  con- 
sidered to  lie  between  40°  and  45°  F.  for  most  of  the 
plants  grown  as  crops  in  this  country. 

The  following  table  shows  minimum,  optimum,  and 
maximum  temperatures  of  growth  for  a  few  plants. 


Minimum. 

Optimum. 

Maximum. 

Mustard 

32°  F. 

8l°F. 

99°  F. 

Barley    . 

41 

83-6 

99-8 

Wheat    . 

41 

83-6 

108-5 

Maize 

49 

92-6 

"5 

Kidney  Bean 

49 

92-6 

US 

Melon    . 

65 

91.4 

in 

The  next  table  shows  the  effect  of  soil,  temperature 
upon  the  growth  of  the  root  of  maize. 


I24 


THE  TEMPERATURE  OF  THE  SOIL      [CHAP. 
ROOT  GROWTH  OF  MAIZE  IN  24  HOURS. 


Temperature. 

Millimetres. 

63°  F. 

1-3 

79 

24-5 

92 

39 

93 

55 

101 

25-2 

108-5 

5-9 

The  osmotic  absorption  of  water  by  the  roots  of 
a  plant  is  much  affected  by  the  temperature  of  the  soil ; 
although  some  plants,  like  cabbage,  will  continue  to  take 
in  a  little  water  even  near  freezing  point,  others  require 
a  higher  temperature ;  for  example,  Sachs  has  shown 
that  tobacco  and  vegetable  marrow  plants  will  wilt 
even  at  night,  when  transpiration  is  very  small,  if  the 
temperature  of  the  soil  falls  below  about  40°  F. 

The  killing  of  plants  like  rose  trees  during  frost  is 
generally  due  to  drying  out  from  this  cause  rather  than 
to  the  actual  cold.  As  the  air  is  often  very  dry  during 
a  frost,  evaporation  continues,  especially  if  a  wind  be 
blowing  at  the  same  time ;  thus  the  exposed  shoots  of 
the  plant  are  losing  water  which  is  not  being  replaced 
by  the  roots,  whose  action  is  suspended  by  the  low 
temperature.  A  covering  of  snow  or  dead  leaves,  or 
even  the  protection  afforded  by  a  little  straw,  bracken, 
or  spruce  boughs,  prevent  the  destruction  of  the  plant, 
not  by  keeping  it  so  much  warmer,  but  by  pro- 
tecting it  from  evaporation. 

The  connection  between  soil  temperature  and  vital 
processes  is  perhaps  most  apparent  in  the  case  of 
germination,  for  which  not  only  is  a  certain  minimum 
temperature  requisite,  but  for  several  degrees  above  this 
minimum  the  germination  is  so  slow  and  irregular  that 
the  young  plant  is  very  liable  to  perish  while  remaining 


v.] 


GERMINATION 


125 


in  such  a  critical  condition.  The  following  table  shows 
the  range  of  temperatures  for  the  germination  of  various 
cultivated  plants. 

TEMPERATURES  OF  GERMINATION. 


FAHRENHEIT. 

Minimum. 

Optimum. 

Maximum. 

Wheat       . 

32°  to  41° 

77°  to  88° 

88°  to  110° 

Barley 

40° 

77°  to  88° 

100°  to  no0 

Oats  . 

32°  to  41° 

88°  to  100° 

Pea  . 

38°  to  41° 

Scarlet  Runner 

49° 

91° 

115° 

Maize 

49° 

91° 

"5° 

Cucumber,  Melon,  etc. 

60°  to  65° 

88°  to  99° 

IIO°   to    120° 

The  practical  bearing  of  these  figures  is  obvious ; 
it  is  necessary  to  sow  some  seeds,  like  the  melon,  in 
heat,  and  to  defer  the  seeding  of  other  crops,  like 
mangolds  or  maize,  until  the  ground  has  acquired  not 
only  the  temperature  necessary  for  germination  but 
one  that  will  ensure  a  subsequent  rapid  growth  of 
the  seedling  plants. 

For  example,  turnips  will  germinate  at  almost  as 
low  a  temperature  as  barley,  but  the  optimum  tempera- 
ture is  higher  for  turnips  ;  they  are  therefore  sown  much 
later  in  the  spring,  when  the  ground  has  more  nearly 
reached  this  temperature,  because  the  seed  is  small  and 
the  young  plant  very  susceptible  to  insect  attacks,  so 
that  the  turnip  seed  must  germinate  and  grow  away 
rapidly  if  it  is  to  succeed. 

Under  ordinary  field  conditions  much  of  the 
nutrition  of  the  crop  depends  upon  the  activity  of 
certain  bacteria  in  the  soil  which  break  down  organic 
compounds  containing  nitrogen,  and  ultimately  resolve 
them  into  the  nitrates  taken  up  by  the  plant.  Most 


126  THE  TEMPERATURE  OF  THE  SOIL      [CHAP. 

bacteria  are  active  within  about  the  same  limits  of 
temperature  as  have  been  indicated  above  for  the 
higher  plants ;  the  nitrification  bacteria,  for  example, 
cease  their  work  below  41°  F.  and  above  130°  R,  their 
optimum  temperature  being  about  99°  F. 

The  way  a  low  temperature  will  check  the  production 
of  nitrates  until  they  are  inadequate  for  the  needs  of 
the  crop  is  often  seen  in  spring,  and  may  be  connected 
with  the  yellow  colour  of  the  young  corn  during  a  spell 
of  cold  and  drying  east  wind. 

Radiation. 

The  main  source  of  the  soil  warmth  consists  in  the 
heat  received  from  the  sun  by  radiation  ;  this,  according 
to  Langley,  amounts  to  about  1,000,000  calories  per 
hour  per  square  metre  of  surface  from  a  vertical  sun  in 
a  clear  sky.  Supposing  this  energy  were  wholly 
absorbed  by  a  layer  of  dry  soil  10  cm.  thick,  its 
temperature  would  rise  by  as  much  as  90°  F.  in  an  hour. 
Of  course  in  nature  many  other  factors  are  at  work  to 
reduce  this  temperature ;  the  sun  is  rarely  vertical, 
the  soil  material  does  not  completely  absorb  but  reflects 
some  of  the  sun's  rays  unchanged  ;  at  the  same  time  it 
is  always  radiating  in  its  turn  rays  of  lower  pitch  than 
the  majority  of  those  received.  The  latter  rays  are 
easily  caught  by  many  substances,  glass  and  water 
vapour  in  particular,  which  are  transparent  to  the  rays 
of  higher  refrangibility  proceeding  from  the  sun.  A 
greenhouse,  for  example,  is  practically  a  radiant  heat 
trap ;  the  temperature  inside  runs  up  because  the  sun's 
rays  of  light  and  heat  can  penetrate  the  glass,  whereas 
the  obscure  heat  rays  radiated  back  again  from  the 
warmed-up  surfaces  inside  the  house  are  not  able  to 
pass  through  the  glass  again.  Just  in  the  same  way  the 
temperature  rises  and  the  sun's  heat  becomes  oppressive 


v.]  COLOUR  AND  TEMPERATURE  127 

when  the  air  is  laden  with  water  vapour,  because  it 
retains  the  radiations  emitted  by  the  surfaces  heated  by 
the  sun.  Per  contra,  the  temperature  of  the  ground 
falls  more  rapidly  at  night  when  the  sky  is  clear  and 
the  air  dry,  for  then  there  is  no  blanket  of  cloud  or 
water  vapour  to  arrest  or  reflect  the  radiations  from  the 
surface. 

The  power  of  soils  to  absorb  the  sun's  rays  depends 
very  much  upon  colour :  with  black  soils  the  absorption 
is  almost  complete  ;  it  is  greater  for  red  than  for  yellow 
soils,  least  of  all  for  those  which  look  distinctly  white  or 
light  coloured.  It  has  already  been  shown  that  the 
colour  of  soils  depends  mainly  upon  humus  and 
hydrated  ferric  oxide,  the  latter  accounts  for  all  the 
red,  yellow,  and  brown  shades,  the  former  for  the  black 
coloration  of  the  soil.  Deep-seated  clays  are  often 
blue  or  green,  due  to  various  ferrous  silicates  or  to 
finely  divided  iron  pyrites,  which  afterwards  oxidise 
to  brown  ferric  oxide.  The  more  finely  grained  a 
soil  is  the  more  surface  it  possesses,  and  the  greater 
amount  of  colouring  matter  that  is  required  to  pro- 
duce a  given  colour ;  a  coarse  sand  is  often  quite 
black  though  it  contains  but  a  small  percentage  of 
humus. 

Though  the  colour  of  a  soil  affects  the  rate  at  which 
it  absorbs  heat,  it  does  not  follow  that  the  dark  soils 
will  lose  with  a  corresponding  rapidity  when  radiation 
is  taking  place  at  night ;  the  emissive  power  of  the 
substance  for  rays  of  low  refrangibility,  such  as  are 
emitted  at  ordinary  temperatures,  is  not  affected  by 
colour.  Hence,  the  extra  heat  gained  by  a  dark  soil 
is  retained  and  not  lost  by  a  corresponding  increase  in 
its  radiating  power. 

The  curves  in  the  accompanying  diagram  (Fig.  10), 
show  the  temperatures  of  the  soil  at  a  depth  of  6 


128  THE  TEMPERATURE  OF  THE  SOIL      [CHAP. 

inches  during  an  April  day  with  a  bright  sun  and  a 
strong  drying  wind.  The  land  was  a  light  loam  of 
a  grey-brown  colour  when  dry ;  it  had  been  culti- 
vated, rolled,  and  the  surface  hoed  over  before  the 
thermometers  were  inserted ;  on  plot  I  the  bare 
ground  was  left  untouched,  plot  2  received  a  dressing 
of  soot  until  the  surface  was  black,  plot  3  was 
similarly  whitened  over  with  lime.  It  will  be  seen 
that  the  covering  of  soot  warmed  the  soil  until  at  3 
P.M.,  when  the  maximum  temperature  was  attained, 
the  difference  was  2-4° ;  this  superiority  is  also  re- 
tained during  the  later  cooling  stages;  even  at  9  P.M. 
the  blackened  soil  was  still  2-5°  warmer  than  the  bare 
ground.  The  whitening  with  lime  had  caused  so  con- 
siderable a  reflection  of  the  radiant  heat  that  the  soil 
beneath  was  always  2  to  3°  cooler  than  the  bare  ground. 
In  carrying  out  this  experiment  it  is  necessary  to  use  no 
more  lime  or  soot  than  will  distinctly  colour  the  soil ; 
the  results  will  be  disturbed  if  an  excess  of  either  loose 
powder  acts  as  a  mulch. 

Specific  Heat. 

The  specific  heat  of  the  substances  of  which  soil 
is  composed  is  comparatively  low,  ranging  from  01 
to  02,  i.e.,  only  from  one-tenth  to  one-fifth  as  much 
heat  will  be  necessary  to  raise  the  temperature  of  I 
Ib.  of  dry  soil  by  i°,  as  would  be  required  to  produce 
the  same  rise  of  temperature  in  an  equal  weight  of 
water.  The  humus  possesses  the  greatest  specific  heat 
and  the  sand  the  least ;  against  this  must  be  set  off  the 
fact  that  the  densities  of  these  soil  constituents  vary 
in  the  opposite  sense,  so  that  the  amounts  of  heat 
required  to  bring  about  a  given  rise  of  temperature  to 
a  certain  depth  in  different  soils  are  more  nearly  equal. 
The  specific  heats  are,  however,  small  in  every  case 


H- 


•'  / 


a 

4i- 

•7 
/ 


\ 


\ 


\\ 


\ 


x. 

W 


HEAT  REQUIRED  TO  WARM  SOIL 


129 


when  compared  with  that  of  water ;  hence  soils  which 
retain  much  water  will  require  far  more  heat  to  raise 
their  temperature  than  dry  soils  would.  In  consequence, 
clay  and  humus  soils  are  cold  because  the  water  they 
retain  gives  them  a  high  specific  heat,  they  require  more 
of  the  sun's  rays  in  spring  to  bring  them  up  to  the 
proper  temperature  for  growth,  while  sandy  and  other 
open-textured  soils  are  warm  because  of  their  dryness. 

If  the  figures  given  by  Oemler  for  the  specific  heats 
of  various  soils  be  combined  with  their  approximate 
densities  and  with  their  minimum  capacity  for  water,  the 
following  results  are  obtained  for  the  specific  heats  of 
certain  typical  soils  in  a  saturated  but  completely  drained 
condition — 


SPECIFIC  HEAT. 

Equal  Weights. 

Equal  Volumes. 

Dry. 

Dry. 

Wet. 

Water 

IO 

1-0 

IO 

Humus 

0-21 

Sandy  Peat 

O«I4 

O-II 

0-72 

Loam 

0-15 

01  8 

o-53 

Clay. 

0-14 

0-15 

0-61 

Sand  . 

0-1 

0-125 

o-34 

The  sandy  soil  only  requires  about  half  as  much 
heat  to  raise  its  temperature  by  a  given  amount  as 
would  be  needed  by  the  peaty  or  clay  soil,  when  all  the 
soils  are  in  a  wet  but  thoroughly  drained  condition ;  of 
course  if  the  clay  or  peat  were  inadequately  drained,  so 
that  a  higher  proportion  of  water  was  retained,  their 
specific  heats  would  approximate  still  nearer  to  that 
of  water. 

Just  as  a  clay  soil  is  slow  to  warm  in  the  spring,  its 
high  specific  heat  causes  it  to  cool  correspondingly 

I 


130  THE  TEMPERATURE  OF  THE  SOIL      [CHAP. 

slowly  after  the  heat  of  the  summer.  On  clay  soils 
growth  will  be  noticed  to  continue  later  into  the  autumn 
than  on  the  lighter  lands. 

Heat  required  for  Evaporation. 

The  coldness  of  a  wet  and  undrained  soil  is  due,  not 
only  to  its  high  specific  heat,  but  to  the  fact  that  so  much 
of  the  heat  it  receives  is  spent  in  evaporating  some  of  its 
retained  water,  without  causing  any  rise  in  temperature. 
The  evaporation  of  I  Ib.  of  water  at  62°  F.,  i.e.,  its  con- 
version into  water  vapour  at  the  same  temperature, 
requires  as  much  heat  as  would  raise  the  temperature  of 
1050  Ibs.  of  water  by  i°  F.,  and,  if  there  be  no  source  of 
external  heat  bringing  about  the  evaporation,  the  sub- 
stance from  which  the  water  is  evaporated  must  become 
cooled  to  a  corresponding  extent  The  cooling  effect  of 
evaporation  is  well  known,  but  its  application  to  the 
soil  is  not  always  realised ;  clays  and  even  more  so  un- 
drained soils  are  cold  and  late,  not  only  because  of  their 
high  specific  heat,  but  because  they  retain  so  much 
water  which  can  be  evaporated.  The  drying  winds  of 
early  spring  exercise  a  great  cooling  effect  whenever 
the  soil  moisture  is  allowed  to  evaporate  freely,  hence 
the  importance  of  establishing  a  loose  tilth,  if  the 
seed  bed  is  to  warm  up  the  temperatures  requisite  for 
germination. 

Anything  providing  a  little  shelter  to  check  evapora- 
tion and  break  the  force  of  the  wind  in  the  spring  will 
have  a  considerable  effect  in  raising  the  soil  temperature. 
The  dotted  curve  in  Fig.  10  shows  the  effect  of  enclosing 
a  plot  of  the  same  land  with  a  slight  hedge  made  of 
spruce  fir  boughs  about  2  feet  high.  In  the  morning 
the  temperature  of  the  sheltered  plot  was  below  that  of 
the  open  ground  because  of  the  shading  from  the  direct 
rays  of  the  sun,  but  as  soon  as  this  effect  was  over 


v.]  EVAPORATION  COOLS  THE  LAND  131 

it  will  be  seen  that  the  wind  break,  by  checking  evapora- 
tion, maintained  the  soil  temperature  more  than  2° 
above  that  of  the  open  ground.  Sufficient  attention  is 
not  given  in  practice  to  the  value  of  even  slight  wind 
breaks  for  checking  evaporation  and  so  raising  the 
temperature  of  the  soil  in  early  spring.  The  raisers  of 
specially  early  vegetables,  radishes  in  particular,  on  a 
strip  of  light  land  close  to  the  sea  in  Kent  are,  however, 
in  the  habit  of  breaking  the  sweep  of  wind  across  their 
fields  by  erecting  temporary  fences  of  lightly  thatched 
hurdles. 

Even  the  stones  upon  the  surface  of  the  land  help. 
In  the  Journal  of  the  Royal  Agricultural  Society  for 
1856,  an  experiment  is  described  in  which  the  flints 
were  picked  off  the  surface  of  one  plot  of  ground  and 
scattered  over  an  adjoining  plot,  with  the  result  that  the 
plot  with  double  its  usual  allowance  of  stones  was  three 
or  four  days  earlier  to  harvest  than  the  rest  of  the  field, 
while  the  plot  without  stones  was  a  week  later  still.  It 
will  always  be  noticed  how  the  grass  upon  a  field  coated 
with  dung  starts  earlier  into  growth,  because  the  loose 
manure  acts  as  a  mulch  and  protects  the  soil  from  the 
cooling  due  to  evaporation. 

Land  which  is  protected  from  evaporation,  and  to 
some  extent  from  radiation,  by  a  layer  of  vegetation,  is 
always  both  warmer  and  less  subject  to  fluctuations  of 
temperature  than  bare  soil. 

The  warming  up  of  a  well-tilled  surface  soil  is 
increased  by  the  fact  that  the  conduction  of  heat  into 
the  soil  below  is  much  checked  by  a  loose  condition.  A 
solid  body  will  always  conduct  heat  far  better  than 
the  same  substance  in  the  state  of  powder,  and  the 
more  compressed  the  powder  is  the  better  it  will  conduct, 
simply  because  there  are  more  points  of  contact.  Hence 
a  rolled  and  tightened  soil  will  conduct  the  heat  it 


132 


THE  TEMPERATURE  OF  THE  SOIL      [CHAP. 


receives  more  rapidly  to  the  lower  layers  than  one 
which  is  loose  and  pulverulent  King  has  shown  that, 
despite  the  increased  evaporation,  there  is  always  a 
higher  temperature  below  a  rolled  than  an  unrolled 
surface. 

A  few  observations  may  be  given  showing  the  effect 
of  drainage  in  enabling  the  sun's  heat  to  raise  the 
temperature  of  soil.  The  curves  (Fig.  11)  show  the 
hourly  temperatures  of  the  drained  and  undrained  por- 
tions of  a  peat  bog  during  two  last  days  in  June  (Parkes, 
J.  R.  Ag.  Soc.,  1844,  142),  at  depths  of  7  inches  and  13 
inches  respectively ;  the  sudden  rise  of  temperature 
between  3  and  4  P.M.  on  the  second  day  was  due  to  a 
thunderstorm,  during  which  heavy  rain  at  a  temperature 
of  78°  F.  was  falling. 

The  figures  in  the  table  below  are  derived  from 
observations  made  by  Bailey-Denton  in  1857  (/.  R. 
Ag.  Soc.,  1859,  273),  on  a  stiff  clay  soil  situated  on  the 
Gault  at  Hinxworth,  the  drains  being  4  feet  deep  and 
25  feet  apart  in  the  drained  part.  It  is  noteworthy 
that  the  temperature  of  the  air  9  inches  above  the 
surface  is  higher  for  the  drained  than  for  the  undrained 
land,  thus  supplying  further  evidence  of  the  cooling 
effect  of  evaporation. 

MEAN  TEMPERATURE  °F.  AT  9  A.M. 


A.  —  Land  Drained. 

B.—  Land  Undrained. 

Air. 

At  18". 

At  42". 

Air. 

At  18". 

At  -42". 

March     .... 
April        .... 
May        .... 

Mean  excess  over  B.    . 

39-4 
43 
52-9 

40-6 
46 
51 

41-7 
44-8 
48-4 

39 
42-4 

52-7 

38-2 

44 
48-8 

40-3 
43-8 
47-1 

0-4 

2-2 

1-2 

•rf  .S 


[To  face  page  132. 


V.]  SPRING  FROSTS  133 

Effect  of  Situation  and  Exposure. 

Other  conditions  being  equal,  in  the  northern  hemi- 
sphere the  soil  temperatures  will  always  be  higher  on 
land  sloping  toward  the  southern  quadrant  than  with 
any  other  aspect.  King  found  a  difference  of  about 
3°  F.  down  to  the  third  foot  between  a  stiff  red-clay  soil 
with  a  southern  slope  of  18°  and  the  same  soil  on  the 
flat;  Wollny  obtained  a  mean  difference  of  i°-5  between 
the  north  and  south  sides  of  a  hill  of  sandy  soil  inclined 
at  15°.  The  chief  cause  of  these  differences  is  the  fact 
that  in  this  country  the  sun  is  never  vertical,  hence  a 
beam  of  sunlight  represented  by  xy>  Fig.  12,  is  spread 


FlG.  12. — Distribution  of  the  Sun's  Rays  on  Southerly  and 
Northerly  Slopes. 

over  an  area  represented  by  AB  when  the  ground  is 
flat ;  if  the  ground  slopes  to  the  south,  the  same  beam  is 
spread  over  the  smaller  area  represented  by  AC  ;  if  the 
ground  slopes  to  the  north,  it  is  spread  over  the  larger 
area  represented  by  AD.  During  the  winter  half-year, 
also,  the  southern  slope  will  have  a  longer  duration  of 
sunlight  than  the  northern  slope. 

Though  in  a  general  way  the  temperature  both  of 
the   air   and   the   soil   decreases   with   elevation    above 


134  THE  TEMPERATURE  OF  THE  SOIL       [CHAP* 

sea-level,  yet  it  is  well  known  that  the  severest  frosts 
occur  locally  at  the  bottom  of  valleys  and  hollow  places. 
This  is  particularly  noticeable  in  the  sudden  night  frosts, 
characteristic  of  early  autumn  and  late  spring,  which 
are  so  dangerous  to  vegetation ;  it  is  usual  to  find  the 
tenderer  plants  of  our  gardens,  such  as  dahlias,  cut 
down  by  frost  on  the  lower  levels  long  before  the 
gardens  on  the  hill  are  affected.  Spring  frosts,  again, 
will  often  nip  the  early  potatoes  in  the  valleys  when 
the  higher  lands  are  untouched.  Fruit  plantations 
should  not  be  set  in  the  valleys,  for  no  crop  suffers 
more  from  these  unseasonable  snaps  of  cold  ;  so  clearly 
is  this  fact  recognised,  that  in  some  fruit-growing 
districts  only  land  above  a  certain  elevation  is  regarded 
as  suitable  for  fruit,  and  commands  a  higher  rent  in 
consequence.  Two  causes  co-operate  in  producing  the 
excess  of  cold  at  the  lower  levels.  The  night  frosts 
in  question  are  always  the  result  of  excessive  radiation 
when  the  sky  is  clear  and  the  air  still.  The  ground 
surface  loses  heat  rapidly  and  cools  the  layer  of  air 
above ;  the  cold  air  thus  produced  is  denser,  and  pro- 
ceeds to  flow  downhill  and  accumulate  at  the  lower 
levels.  There  is  thus  a  renewal  of  the  air  above  the 
higher  slopes,  and  the  effect  of  radiation  is  mitigated 
by  the  inflow  of  warmer  air ;  at  the  bottom  no  change 
of  air  is  produced  and  the  radiation  proceeds  to  its 
full  effect. 

At  the  same  time  the  vegetation  in  the  valley 
is  generally  more  susceptible  to  a  frost ;  the  greater 
warmth  by  day,  together  with  the  extra  moisture  and 
shelter,  induce  an  earlier  and  a  softer  growth. 

The  other  cause  that  operates  to  produce  severer 
frosts  in  the  valleys  arises  from  the  fact  that  frosts, 
and  radiation  weather  generally,  accompany  the  dis- 
tribution of  pressure  known  as  an  "  anti-cyclone,"  during 


vMax. 


May  llth          12th  13th  Mth  15th  May  HJth,  11HVJ 

FlG.   13. — Temperatures  (Maximum  and  Minimum)  at  Various  Altitudes. 
A  =  100  feet.         B  =  150  feet.         C  =  500  feet. 


[  To  t«ce  page  135. 


V.]  DRY  SOILS  ARE  EARLY  135 

which  there  is  always  a  gentle  downdraught  of  cold 
air  from  the  clear  sky  above.  But  the  circulation  of 
air  in  an  anti-cyclone  is  always  reversed  at  a  certain 
elevation,  so  that  as  one  ascends,  the  downdraught  of 
cold  air  becomes  less  and  eventually  ceases ;  the  mean 
temperature  at  the  same  time  rises  instead  of  falling 
with  the  elevation. 

Observations  of  the  minimum  temperature  on  the 
grass  made  at  two  stations  on  a  gentle  slope  of  the  downs 
at  Wye,  about  a  mile  apart  and  differing  in  level  by 
100  feet,  showed  during  a  period  of  thirty-six  days 
in  February  and  March  1902  (which  was  exceptionally 
calm  and  mild),  a  mean  difference  of  i°  in  favour  of 
the  upper  station,  but  on  two  occasions  the  lower 
thermometer  fell  to  24°- 5  and  2i°-5,  when  the  upper 
thermometer  was  30°-  5  and  29°-  5  respectively. 

The  accompanying  curves  (Fig.  13)  show  the  daily 
maxima  and  minima  of  the  air  temperatures  of  4  feet 
from  the  ground  for  a  few  days  before  and  after  the 
occurrence  of  a  disastrous  late  spring  frost  in  May  1902. 
One  station,  A,  was  in  a  river  meadow  about  100  feet 
above  sea-level,  the  second,  B,  was  on  a  terrace  about 
50  feet  higher  and  half  a  mile  away ;  the  third,  C,  was  on 
the  flat  summit  of  the  down,  500  feet  above  sea-level 
and  about  i£  mile  from  the  first  station.  It  will  be 
seen  that  the  river-side  station  gave  always  the  highest 
maxima  during  the  period  and  the  lowest  minima, 
showing  on  the  one  occasion  1 1  °  of  frost. 

Early  and  Late  Soils. 

From  all  the  considerations  developed  above  it  will 
be  seen  that  an  early  soil  is  essentially  a  coarse-textured 
and  well-drained  one.  Such  a  soil  retains  little  water, 
thus  possessing  a  low  specific  heat,  and  is  easily  warmed  ; 


136  THE  TEMPERATURE  OF  THE  SOIL       [CHAP. 

at  the  same  time  the  low  water  content  gives  rise  to 
less  evaporation  at  the  surface,  and  this  great  cause  of 
cooling  is  minimised.  The  dryness  of  the  soil  permits 
of  early  cultivation,  which,  by  cutting  off  the  access  of 
subsoil  water  and  diminishing  the  conduction  of  heat 
from  the  surface,  quickens  the  warming  up  of  the 
seed  bed.  The  early  aeration  and  warming  of  the  soil 
promotes  the  nitrification  which  is  also  necessary  to 
growth.  If,  further,  the  soil  be  stony,  the  conduction  of 
heat  from  the  surface  layer  into  the  soil  is  more  rapid, 
solids  being  better  conductors  than  powders.  Such 
soils,  again,  are  generally  dark  coloured,  because  on  the 
comparatively  small  surface  exposed  by  the  coarse 
grains  the  same  proportion  of  humus  has  a  greater 
colouring  effect 

These  conditions  are  generally  fulfilled  by  the 
alluvial  soils  bordering  the  larger  rivers ;  in  the 
neighbourhood  of  large  towns,  which  are  generally 
situated  on  a  river,  they  form  the  typical  market- 
gardening  soils,  especially  as  their  natural  poverty  can 
be  alleviated  by  the  large  supply  of  dung  which  is  easily 
obtainable  from  the  town. 

At  the  same  time  these  soils  have  their  dis- 
advantages ;  from  both  their  nature  and  their  situation 
they  are  subject  to  rapid  changes  of  temperature  ;  they 
suffer  much  from  night  frosts  both  in  spring  and 
autumn,  and  dry  out  easily  in  the  summer,  so  that 
some  crops  do  not  come  to  their  full  growth.  Autumn 
planted  vegetables  grow  away  rapidly,  and  are  apt  to 
become  "  winter  proud  "  and  killed  by  severe  weather. 

Of  course,  to  ensure  the  maximum  of  earliness  and 
freedom  from  spring  frosts,  the  geographical  situation 
and  the  climate  must  be  considered  as  well  as  the  nature 
of  the  soil.  The  neighbourhood  of  the  sea  or  any  large 
body  of  water  has  a  great  effect  in  equalising  the 


v.]  EARLY  AND  LATE  SOILS  137 

temperatures  and  preventing  severe  frosts ;  in  the 
British  Islands,  for  example,  the  earliest  potatoes  are 
grown  in  Jersey,  near  Penzance,  and  on  other  light  land 
along  the  southern  coast  of  Cornwall,  and  again  a  little 
later  near  the  sea  in  Ayrshire.  Light  land  round  the 
coasts  of  Kent  and  Essex,  which  borders,  and  in  some 
cases  is  almost  surrounded  by  the  sea,  is  also  specially 
valued  for  the  growth  of  early  vegetables. 

The  soils  naturally  retentive  of  water  are  late, 
both  because  they  dry  slowly  and  are  rarely  fit  to  work 
early  in  the  year,  and  because  the  high  water  content 
keeps  their  temperature  down.  Except  in  long-con- 
tinued droughts  they  maintain  a  supply  of  water  to  the 
plant,  their  high  specific  heat  keeps  them  at  a  com- 
paratively equable  temperature  and  prevents  them  from 
cooling  down  so  soon  when  the  summer  heats  are 
past.  In  consequence,  the  crop  is  neither  forced  early 
to  maturity  nor  is  growth  checked  so  soon  in  the 
autumn,  the  period  of  development  is  prolonged  until 
in  some  cases  the  season  becomes  unsuitable  for 
ripening.  Many  subtle  differences  may  be  noticed 
between  the  quality  of  produce  grown  upon  early  and 
late  soils ;  for  example,  a  comparison  made  by  the 
author  of  the  same  variety  of  apple  grown  upon 
adjoining  clay  and  sandy  soils  showed  that  the  apples 
from  the  clay  land  were  smaller  and  greener,  but 
contained  a  greater  proportion  of  sugar  and  acid, 
and  possessed  a  higher  aroma  than  the  apples  grown 
upon  the  lighter  and  earlier  soil.  Wheat  grown  on 
the  clays  is  generally  of  better  quality  and  "  stronger  " 
than  that  yielded  by  the  lighter  soils  ;  whereas  the 
lighter  soils  yield  the  finer  barley,  in  which  carbo- 
hydrates and  not  nitrogenous  materials  are  characteristic 
of  high  quality.  On  a  light  soil,  becoming  both  warm 
and  dry  early  in  the  season,  the  plant  ceases  the  sooner 


138         THE  TEMPERATURE  OF  THE  SOIL     [CHAP.  V. 

to  draw  nutrient  material  from  the  soil ;  assimilation, 
however,  continues  after  the  plant  has  ceased  to  feed ; 
finally,  maturation  —  the  removal  of  the  previously 
elaborated  material  into  the  seed — begins  earlier  and 
can  be  more  thoroughly  accomplished.  Hence  grain 
from  an  early  soil  is,  on  the  whole,  characterised  by 
a  higher  proportion  of  carbohydrate  to  albuminoid, 
always  provided  that  no  excessive  or  premature  dry- 
ing-out has  taken  place,  for  that  ripens  off  the  grain 
before  the  transference  of  starch  has  been  completed. 


CHAPTER  VI 

THE  CHEMICAL  ANALYSIS  OF  SOILS 

Necessary  Conventions  as  to  the  Material  to  be  Analysed — 
Methods  Adopted  —  Interpretation  of  Results  —  Distinction 
between  Dormant  and  Available  Plant  Food — Analysis  of 
the  Soil  by  the  Plant— Determination  of  "  Available  "  Phos- 
phoric Acid  and  Potash  by  the  Use  of  Weak  Acid  Solvents. 

THE  chemical  analysis  of  a  soil  aims  at  ascertaining  the 
amount  which  the  soil  contains  of  the  various  elements 
necessary  to  the  nutrition  of  the  plant,  with  a  view  of 
either  making  good  the  general  deficiencies  of  the  soil  or 
of  adjusting  the  supply  of  plant  food  to  such  special 
requirements  of  a  particular  crop  as  may  have  been 
indicated  by  previous  experiment. 

The  analysis  of  plants  grown  under  ordinary  con- 
ditions shows  that  a  comparatively  limited  number  of 
elements  enters  into  their  composition ;  in  the  main  they 
are  composed  of  water  and  certain  combustible  com- 
pounds of  carbon,  hydrogen,  nitrogen,  and  sulphur.  In 
the  mineral  residue  that  is  left  after  the  combustible 
material  has  been  burnt  off,  will  be  found  potassium, 
sodium,  calcium,  magnesium,  and  a  little  iron  among 
bases ;  and  phosphorus,  chlorine,  sulphur,  and  silicon 
among  non-metallic  elements.  Manganese  in  very  small 
quantities  occurs  in  nearly  all  plants :  other  elements 
like  lithium,  zinc,  copper,  are  found  in  traces  under 

189 


140         THE  CHEMICAL  ANAL  YS1S  OF  SOILS    [CHAP. 

special  conditions  of  soil.  By  pot  cultures  in  the 
laboratory  it  can  be  shown  that  of  the  ;ibove  elements, 
the  carbon,  hydrogen,  and  oxygen  are  drawn  from  the 
atmosphere  or  the  water,  and  that  nitrogen,  chlorine, 
sulphur,  phosphorus,  among  non-metals,  and  potassium, 
calcium,  magnesium  and  iron,  among  metals,  are  ele- 
ments indispensable  to  the  plant,  and  are  derived  by 
way  of  the  root  from  the  soil.  In  view  of  the  above 
facts  it  is  clearly  unnecessary  to  make  an  ultimate 
determination  of  all  the  elements  present  in  the 
soil,  which  has  already  been  shown  to  consist  largely 
of  sand  and  various  silicates  of  alumina,  etc.  These 
materials  constitute  the  medium  in  which  the  plant 
grows,  but  do  not  themselves  supply  it  with  any 
food ;  they  need  not,  therefore,  be  estimated  chemi- 
cally. 

The  chemical  analysis  of  a  soil,  then,  resolves  itself 
into  determinations  of  the  nitrogen,  phosphorus,  potas- 
sium, calcium,  and  (of  less  importance)  of  sodium,  mag- 
nesium, iron,  aluminium,  chlorine,  and  sulphur.  To 
these  must  be  added  the  determination  of  the  carbon 
compounds  of  the  soil,  which  have  already  been  touched 
on  under  the  head  of  humus,  and  of  the  carbonates  of 
calcium  and  magnesium,  which  in  most  soils  constitute 
the  bases  available  for  neutralising  any  acids  that  may 
be  produced.  Having  decided  upon  the  elements  to  be 
determined  it  would  then  be  possible  to  proceed  as  in 
an  ordinary  mineral  analysis :  the  sample  of  soil  would 
be  reduced  to  such  a  state  of  division  as  would  admit 
of  drawing  an  accurate  small  sample,  and  then  entirely 
disintegrated  by  some  such  reaction  as  fusion  with 
ammonium  fluoride.  But  results  obtained  in  this  way 
would  give  very  imperfect  information  about  the  soil,  for 
the  procedure  draws  no  distinction  between  material 
present  in  the  unweathered  interior  of  the  stones  and 


vi.]  NECESSAR  Y  CONVENTIONS  141 

coarser  particles,  which  could  not  reach  the  plant  for 
generations,  and  that  which  exists  as  very  small  particles 
or  as  a  coating  on  the  larger  ones,  and  is  therefore  open 
to  attack  by  the  water  in  the  soil.  The  nutrient  material 
of  the  soil  can  only  reach  the  plant  in  the  dissolved 
state,  and  in  dealing  with  slightly  soluble  substances 
such  as  constitute  the  soil,  the  amount  which  goes  into 
solution  is  practically  proportional  to  the  surface 
exposed.  But  the  surface  exposed  increases  as  the 
material  is  subdivided,  one  gram  of  soil  in  pieces 
i  mm.  in  diameter  would  only  expose  one-thousandth 
of  the  surface  exposed  by  the  same  amount  of  soil  in 
particles  oooi  mm.  in  diameter,  so  that  to  all  intents 
and  purposes  the  stones  and  coarser  particles  con- 
tribute such  a  small  proportion  of  the  surface  of  the 
soil  that  the  material  dissolved  from  them  can  be 
neglected. 

For  these  reasons — the  small  surface  exposed  by 
the  larger  particles  and  the  unweathered  nature  of 
the  compounds  within  them  —  the  stones  above  a 
certain  size  are  not  included  in  the  analysis,  nor  is 
any  attempt  made  to  bring  into  complete  solution  even 
the  selected  material.  Hence  it  becomes  necessary 
in  soil  analysis  to  accept  certain  "conventions"  as  to 
the  preparation  of  the  soil  for  analysis,  the  nature  of  the 
acid  used  for  solution,  and  the  duration  and  temperature 
of  the  attack ;  all  of  which  factors  so  affect  the  mineral 
matter  going  into  solution  that  results  are  only  com- 
parable when  obtained  in  the  same  way.  It  must 
always  be  remembered  that  soil  analysis  is  only  a 
relative  process,  by  which  soils  that  are  unknown  can 
be  compared  with  others  whose  fertility  has  been 
tested  by  experience ;  no  means  exist  of  directly 
translating  the  results  into  terms  of  the  crop  the  soil 
will  carry.  The  methods  of  analysis  that  are  indicated 


142         THE  CHEMICAL  ANAL  YSIS  OF  SOILS     [CHAP. 

below  are  those  adopted  by  the  members  of  the 
Agricultural  Education  Association  in  this  country : 
unfortunately,  there  is  no  uniformity  in  the  methods 
pursued  even  among  chemists  in  the  same  country, 
wide  as  are  the  variations  introduced  by  the  different 
processes  in  vogue.  For  example,  an  acid  such  as 
hydrochloric  will  dissolve  very  different  amounts  of 
potash  from  a  given  soil,  according  as  the  soil  is 
treated  directly  with  the  acid  or  first  ignited,  nor  is  there 
any  constant  relation  between  the  amount  dissolved 
from  ignited  and  from  raw  soil. 

Method  of  Analysis. 

The  soil  sample  is  taken,  passed  through  the  3  mm. 
sieve,  and  air-dried,  exactly  as  previously  described 
for  the  mechanical  analysis.  From  the  large  air-dried 
sample  of  "fine  earth"  a  portion  of  about  100  grams 
is  drawn,  and  either  ground  in  a  steel  mill  or  broken 
in  a  steel  mortar  till  it  will  all  pass  through  a  sieve 
with  round  holes  i  mm.  in  diameter.  This  is  done 
to  enable  the  analyst  to  draw  a  fair  sample  weighing 
only  a  few  grams :  if  the  "  fine  earth "  which  passes 
the  3  mm.  sieve  were  itself  used,  it  would  be  impossible 
to  adjust  the  relative  proportions  of  coarse  and  fine  to 
correspond  with  the  bulk.  It  is  not  uncommon  to  find 
coarse  particles  of  carbonate  of  lime  sparsely  scattered 
through  the  soil  when  the  land  has  been  limed ;  only  by 
grinding  and  mixing  can  this  matter  become  evenly 
distributed  through  the  soil.  On  the  ground  material 
the  following  determinations  are  made : — 

(1)  Moisture  lost  at  ioo°C. 

(2)  Loss  on  ignition. 

(3)  Nitrogen. 

(4)  Earthy  carbonates. 


VI.]  ANALYTICAL  METHODS  143 

(5)  Phosphoric  acid  and  potash  soluble  in  strong 
hydrochloric  acid.  If  necessary,  soda,  lime, 
magnesia,  oxides  of  iron,  alumina,  and 
sulphuric  acid  can  be  determined  in  the  same 
solution. 

(1)  About   5   grams  are  weighed   out  into  a  plati- 
num dish  or  porcelain  basin  and  dried  in  the  ordinary 
steam  oven,  the  temperature  of  which   is   never   quite 
1 00°  C.     If  the   soil   contains  much  organic  matter,  it 
will  be  difficult   to  bring   it  to  a  constant  weight,  the 
material  will  slowly  lose  water  for  weeks.     An  arbitrary 
limit  of  twenty-four  hours  drying  should  be  taken. 

(2)  The  loss  on  ignition  should  represent  the  organic 
matter  which  is  burnt  to  carbon  dioxide  and  water  when 
the   soil   is   heated    in    the   air,  but  it  is  impossible  to 
avoid  at  the  same  time  driving  off  some  of  the  water 
of  constitution   in  the    zeolites,   kaolinite,   and   similar 
hydrated   silicates   in   the   soil.     It  is   difficult   even  to 
obtain   consistent   results,  because  of  variations   in  the 
temperature  and  time  of  the  operation.     The  best  plan 
is  to  heat  the  soil,  as  dried  in  the  previous  operation, 
at  as  low  a  temperature  as   possible,  to  a  barely  visible 
redness,  preferably  in  a  platinum  dish,  for  some  hours 
with  occasional  stirring. 

The  loss  on  ignition  is  wanted  as  a  measure  of  the 
organic  matter  of  the  soil,  but  we  have  no  means  of 
estimating  the  varying  part,  great  with  clay  soils, 
that  is  played  by  the  water  of  constitution.  It  is  possible 
to  get  a  better  measure  of  the  organic  matter  by  estimat- 
ing the  total  carbon  in  the  soil  and  assuming  that  the 
organic  matter  of  the  original  soil  contained  about 
55  per  cent  of  carbon.  The  combustion  of  a  soil  by  the 
ordinary  method  for  determining  carbon  is  rather  a 
tedious  process  even  in  skilled  hands ;  in  dealing  with 
soils  it  is  convenient  to  effect  the  oxidation  by  means  of 


144          THE  CHEMICAL  ANAL  YS1S  OF  SOILS     [CHAP. 

a  mixture  of  sulphuric  and  chromic  acids,  taking  care  to 
interpose  a  tube  of  heated  copper  oxide  between  the 
flask  containing  the  soil  and  acids  and  the  apparatus 
used  for  absorbing  the  carbon  dioxide,  in  order  to 
complete  the  oxidation  of  some  of  the  products  formed. 
This  process  can  be  made  to  follow  the  determination  of 
the  carbon  dioxide  evolved  by  the  action  of  acid  alone, 
the  same  apparatus  and  the  same  portion  of  soil  serving 
for  both. 

(3)  The  nitrogen  in   10  to  20  grams  of  the  ground 
"  fine  earth  "  is  estimated  by  Kjeldahl's  process.    Though 
there   is  some  nitrate  present   in   the   soil,   no  special 
precaution  need  be  taken  on  its  account,  the  proportion 
it  bears  to   the   total   nitrogen   is   so   small   as   to   be 
negligible. 

(4)  The  earthy  carbonates  of  the  soil  are  estimated 
from  the  quantity  of  carbon  dioxide,  which  is  evolved 
on   treating    the    ground    "  fine   earth "   with   an   acid. 
When  the  proportion  of  calcium  carbonate  is  high  the 
determination   can  be  made  by  the  usual  gravimetric 
methods.      Scheibler's    apparatus    for    measuring    the 
volume  of  carbon  dioxide  evolved  is  suitable  when  the 
proportion   of  calcium   carbonate   does   not   fall   below 
i  per  cent. ;  below  that  point  some  other  method  must 
be  employed,  because  all  the  carbon  dioxide  evolved  goes 
into  solution  in  the  reacting  acid.     The  most  exact  and 
convenient  method  for  determining  calcium  carbonate, 
especially  when   the  quantity   involved   is  very  small, 
consists   in   absorbing   the   carbon   dioxide  evolved  by 
dilute  caustic  soda  in  a  Reiset  tower,  and  estimating  the 
carbon  dioxide  by  titrating  the  alkali  first  with  phenol- 
phthalein  and  then  with  methyl  orange  as  an  indicator 
(see  Amos.  Jour.  Agric.  Set.,  I,  1905,  322).     Certain  acid 
soils   rich   in   humus  contain  other  organic   substances 
which  yield  carbon  dioxide  on  boiling  with  dilute  acid, 


vi.]  METHODS  OF  ANALYSIS  145 

in  which  case  the  soil  should  be  attacked  with  a  boiling 
dilute  solution  of  ammonium  chloride.  It  is  not 
sufficient  in  such  cases  to  estimate  the  calcium  dissolved 
by  dilute  acids  from  the  soil,  because  there  are  always 
present  other  compounds  of  calcium,  e.g.,  silicates  and 
sulphates,  which  are  soluble  in  the  acid  and  would  be 
reckoned  as  calcium  carbonate.  The  factor  that  is 
required  is  not  the  calcium,  but  the  amount  of 
carbonate  which  will  serve  as  a  base  in  the  soil  and 
combine  with  the  acids  liberated  by  decay,  nitrifica- 
tion, or  from  some  of  the  artificial  manures.  To  this 
end  it  is  not  necessary  to  discriminate  between  the 
carbonates  of  calcium  and  magnesium,  accordingly 
the  carbon  dioxide  evolved  is  calculated  back  to 
calcium  carbonate.  In  a  few  soils  ferrous  carbonate 
may  be  present ;  this  is  oxidised  to  ferric  hydrate 
when  the  powdered  soil  is  boiled  with  water,  and 
may  be  so  removed  before  determining  the  carbon 
dioxide.  In  temperate  climates,  however,  it  is  only 
a  few  bog  soils  that  need  be  examined  for  ferrous 
carbonate. 

(5)  For  the  determinations  of  soluble  constituents 
20  grams  of  the  powdered  soil  are  placed  in  a  flask 
of  Jena  glass,  covered  with  about  70  c.c.  of  strong  hydro- 
chloric acid,  and  boiled  for  a  short  time  over  a  naked 
flame  to  bring  it  to  constant  strength.  The  acid  will  now 
contain  about  20-2  per  cent,  of  pure  hydrogen  chloride. 
The  flask  is  loosely  stoppered,  placed  on  the  water- 
bath,  and  the  contents  allowed  to  digest  for  about  forty- 
eight  hours.  The  solution  is  then  cooled,  diluted,  and 
filtered.  The  washed  residue  is  dried  and  weighed  as 
the  material  insoluble  in  acids. 

The  solution  is  made  up  to  250  c.c.  and  aliquot 
portions  are  taken  for  the  various  determinations. 
The  analytical  operations  are  carried  out  in  the  usual 

K 


146         THE  CHEMICAL  ANALYSIS  OF  SOILS     [CHAP. 

manner,  but  special  care  must  be  taken  to  free  the 
solution  from  silica  and  organic  matter.  For  phos- 
phoric acid  a  portion  of  the  solution  is  evaporated  to 
dryness  and  ignited,  the  residue  is  taken  up  with 
hydrochloric  acid,  filtered,  again  evaporated  to  dry- 
ness,  and  heated  in  an  air  -  bath  for  half  an  hour  at 
105°.  This  residue  is  then  taken  up  with  dilute 
nitric  acid,  filtered,  and  made  up  to  about  50  c.c. 
Five  grams  of  ammonium  nitrate  are  added,  and  50 
c.c.  of  a  solution  of  ammonium  molybdate  containing 
60  grams  molybdic  acid  per  litre.  The  mixture  is 
put  aside  in  a  warm  place  for  twenty-four  hours,  the 
precipitate  is  filtered  off,  and,  after  washing  with 
ammonium  nitrate  solution,  is  dissolved  by  ammonia 
into  a  tared  porcelain  basin,  evaporated  to  dryness, 
and  gently  ignited  over  an  Argand  burner.  The 
resulting  material  contains  3-794  per  cent,  of  phos- 
phoric acid.  For  the  determination  of  potash  the 
same  procedure  is  followed,  but  the  residue  after  the 
second  evaporation  is  taken  up  with  dilute  hydro- 
chloric instead  of  nitric  acid.  To  the  solution  25  c.c. 
of  a  solution  of  chloroplatinic  acid  containing  0-005 
gram  platinum  per  c.c.  is  added,  and  the  whole  gently 
evaporated  over  a  water-bath  till  almost  dry.  It  is 
then  thrown  on  to  a  filter  and  washed  with  alcohol, 
then  washed  again  with  a  solution  of  ammonium 
chloride  which  has  been  saturated  with  the  double 
chloride  of  platinum  and  ammonium,  and  finally  dis- 
solved off  the  filter  paper  with  a  little  hot  water  in  a 
tared  basin,  evaporated,  and  weighed.  A  Gooch  crucible 
is  most  convenient  for  handling  both  the  phosphoric 
acid  and  potash  precipitates. 

The  other  determinations  which  may  be  made  in 
this  solution  consist  of  soda,  lime,  magnesia,  iron, 
alumina,  manganese,  and  sulphuric  acid,  but  in  most 


vi.]  METHODS  OF  ANALYSIS  147 

cases  these  may  be  omitted.  It  is  occasionally  desirable 
to  examine  the  soluble  salts  in  the  soil ;  about  200 
grams  of  the  fine  earth  should  be  successively  washed 
with  small  portions  of  hot  water  by  the  aid  of  a 
filter -pump.  In  the  solution  the  total  solids  are 
determined ;  they  consist,  in  the  main,  of  the  nitrates, 
sulphates,  and  chlorides  of  sodium,  potassium,  mag- 
nesium, and  calcium,  which  can  be  determined  by 
the  usual  methods.  Of  course,  the  amount  of  soluble 
salts  to  be  found  in  the  surface  soil  at  any  time  is 
largely  regulated  by  the  previous  weather ;  after  con- 
siderable rainfall  the  soluble  salts  are  washed  down 
into  the  subsoil,  after  long  evaporation  they  are  con- 
centrated in  the  surface  layers.  The  amount  of  nitrates 
that  is  present  is  further  affected  by  the  previous 
cropping,  temperature,  and  working  of  the  soil,  and 
by  the  manipulation  the  soil  receives  after  it  reaches 
the  laboratory.  Thus  the  determination  of  the  soil 
constituents  that  are  soluble  in  water  does  not  enter 
into  the  ordinary  routine  of  analysis,  their  presence 
is  affected  by  so  many  temporary  factors  which  pre- 
vent the  comparison  of  one  soil  with  another. 

As,  however,  the  determination  of  the  amount  of 
nitrate  present  in  a  soil  is  often  required  for  other 
purposes,  it  will  be  convenient  here  to  indicate  the 
method  to  be  followed.  In  the  first  place,  the  soil  must 
be  analysed  either  immediately  after  it  has  been  sampled 
and  after  rapid  drying  with  the  aid  of  heat,  for  the 
manipulation  a  soil  sample  usually  receives  in  the 
drying,  sifting,  and  other  preliminary  operations, 
will  cause  the  production  of  large  quantities  of 
nitrates  in  ordinary  soils. 

A  funnel  with  a  large  filtering  surface,  at  least  2 
inches  in  diameter,  must  be  taken  ;  Warington  originally 
made  use  of  the  inverted  upper  portion  of  a  Winchester 


148          THE  CHEMICAL  ANAL  YSIS  OP  SOILS    [CHAP. 

quart  bottle  with  a  disc  of  copper  gauze,  2  inches  in 
diameter  resting  in  the  neck,  but  this  may  be  replaced 
advantageously  by  a  Buchner  funnel  6  inches  in 
diameter.  In  either  case  the  funnel  is  connected  with 
an  exhaust  -  pump,  the  disc  is  covered  with  a  good 
filter  paper  wetted,  then  at  least  500  grams  of  the  soil 
are  packed  carefully  into  the  funnel  and  pressed  down  a 
little,  care  being  taken  to  avoid  plastering  if  the  soil  is 
clayey.  The  soil  sample  as  it  comes  from  the  field  is 
spread  out,  roughly  crumbled,  and  mixed  ;  from  this  the 
500  grams  or  so  are  taken  and  weighed  before  putting 
on  the  funnel.  Another  portion  is  weighed  out  and 
dried  in  the  steam  oven,  to  ascertain  the  proportion  of 
water  in  the  sample.  50  c.c.  of  hot  distilled  water  are 
now  poured  on  the  soil,  allowed  to  stand  a  few  minutes, 
and  the  pump  started.  When  the  liquid  has  been  drawn 
through,' successive  small  portions  of  hot  water  are  put 
on,  and  the  pump  started  afresh ;  it  will  be  found 
possible  to  wash  out  practically  the  whole  of  the  nitrate 
with  100  c.c.  of  water. 

If  the  liquid  shows  any  tendency  to  come  through 
the  filter  turbid,  this  can  be  obviated  by  adding  a  few 
drops  of  sulphuric  acid  to  the  water.  In  the  filtered 
liquid  the  nitrates  may  be  determined  by  reducing  with 
the  zinc  copper  couple,  distilling  off  the  ammonia  and 
determining  it  either  by  Nesslerising  or  by  titration, 
according  to  its  amount.  The  couple  is  prepared  by 
dipping  half  a  dozen  strips  of  thin  sheet  zinc,  6  inches 
long  by  i  \  broad,  successively  into  dilute  caustic  soda, 
very  dilute  sulphuric  acid,  and  then  into  a  3  per  cent, 
solution  of  copper  sulphate,  in  which  it  is  allowed  to 
remain  until  it  has  acquired  a  good  black  deposit  of  copper. 
They  are  washed  by  immersion  in  water,  and  finally  in 
ammonia -free  distilled  water,  and  placed  in  a  bottle 
with  200  c.c.  of  the  soil  extract  and  a  crystal  of  oxalic 


vi.]  ANALYSES  OF  TYPICAL  SOILS  149 

acid.  The  bottle  is  kept  in  a  warm  place  or  an  incubator 
at  25*  for  twenty-four  hours  before  distilling  off  the 
ammonia. 

The  table  (Appendix  I.)  shows  the  analyses  by  the 
method  above  described  of  a  few  typical  soils. 

It  will  be  seen,  as  a  rule,  that  the  water  retained  by 
the  soil  when  air  dry,  the  loss  on  ignition,  and  the 
nitrogen,  rise  and  fall  together,  because  the  humus 
which  contains  the  nitrogen  is  the  most  hygroscopic 
constituent  of  soils.  Clay  soils  which  tend  to  conserve 
humus  also  contain  the  most  constitutional  water ;  this 
further  tends  to  increase  the  loss  on  ignition  in  their  case. 

The  proportion  of  nitrogen  found  ranges  from  0-5 
per  cent,  in  very  rich  pasture  soils  down  to  below  o-i 
per  cent,  on  light  arable  soils,  it  is  rarely  up  to  0-2  per 
cent,  in  arable  soils,  and  the  warmer,  the  more  open, 
and  more  worked  the  soil  is,  the  less  will  be  the  pro- 
portion of  nitrogen.  In  the  fertile  hop-gardens  of 
East  Kent  the  percentage  of  nitrogen  is  rarely  as  much 
as  0-2  per  cent.,  despite  the  great  dressings  of  nitro- 
genous manure  that  are  annually  applied. 

The  proportion  of  phosphoric  acid  in  soils  is  not  so 
variable  as  the  proportion  of  nitrogen ;  it  ranges  from 
about  0-06  per  cent,  to  0-2  per  cent. ;  the  lower  amounts 
occur  generally  on  the  sands  and  clays,  the  higher  on 
loams  and  soils  well  provided  with  calcium  carbonate. 

The  proportion  of  potash  shows  extreme  variations, 
a  clay  soil  may  yield  one  per  cent,  of  potash  to  strong 
hydrochloric  acid,  a  sand  only  one-tenth  as  much.  It 
has  already  been  pointed  out  that  "clay"  is  chiefly 
the  result  of  the  weathering  of  felspars  and  kindred 
minerals  containing  potash ;  this  weathering  is  never 
chemically  complete,  so  that  all  soils  containing  any 
considerable  admixture  of  clay  are  necessarily  rich  in 
potash.  The  amount  dissolved  out  by  hydrochloric 


150        THE  CHEMICAL  ANALYSIS  OF  SOILS      [CHAP. 

acid  is  also  somewhat  of  an  accidental  figure,  as  it 
depends  very  much  on  how  far  the  previous  treatment 
of  the  soil  has  forwarded  the  weathering  process,  for 
there  remains  in  all  soils  rich  in  potash  much  material 
that  will  not  yield  potash  to  strong  hydrochloric  acid 
even  after  forty-eight  hours'  digestion.  For  example, 
the  soil  from  one  of  the  plots  in  the  Broadbalk  wheat- 
field  at  Rothamsted  only  yielded  o  5  per  cent,  of  potash 
to  hydrochloric  acid,  but  when  completely  broken  up 
by  ammonium  fluoride  it  was  found  to  contain  2-26  per 
cent,  of  potash. 

Of  all  the  soil  constituents  calcium  carbonate  shows 
the  widest  fluctuations  ;  it  may  constitute  40  or  50  per 
cent,  of  some  of  the  thin  soils  resting  on  the  chalk, 
or  it  may  sink  on  some  of  the  sands  and  clays  to  such 
small  proportions  as  only  to  be  detected  by  the  most 
refined  analysis. 

The  importance  of  the  calcium  carbonate  lies  not 
in  the  calcium  that  it  supplies  for  the  nutrition  of 
plants,  but  in  that  it  acts  as  the  chief  base,  maintain- 
ing the  neutrality  of  the  soil.  Many  plant  diseases, 
like  the  slime  fungus  which  causes  "  finger-and-toe " 
in  turnips,  etc.,  are  only  prevalent  when  the  soil  is 
losing  its  neutral  condition,  and  are  not  found  when  a 
sufficiency  of  calcium  carbonate  is  present.  The  normal 
changes  in  a  soil  are  brought  about  by  bacteria,  which 
only  flourish  when  the  medium  is  neutral  or  very  faintly 
alkaline ;  as  soon  as  the  soil  becomes  acid  the  bacterial 
actions  are  largely  suspended,  and  in  their  place  moulds 
and  other  micro-fungi  become  predominant.  It  is  for 
this  reason  always  desirable  to  test  the  reaction  of  a 
soil  by  putting  a  little  on  litmus  paper,  moistening 
it,  and  after  a  few  minutes  washing  away  the  soil. 
What  proportion  of  calcium  carbonate  is  required  for 
fertility  and  health  is  difficult  to  say,  probably  an 


vi.]  INTERPRE TA  T/ON  OF  RES ULTS  i  > i 

inferior  limit  of  0-5  per  cent,  is  the  lowest  that  is  safe. 
In  the  case  of  soils  containing  about  this  proportion 
much  will  depend  on  how  finely  it  is  disseminated, 
0-5  per  cent,  in  visible  pieces  will  not  be  so  effective 
as  o- 1  per  cent,  of  the  amount  in  particles  of  the  same 
order  of  size  as  the  clay  or  silt  particles.  For  this 
reason  it  is  advisable  when  analysing  a  doubtful  soil  of 
this  kind,  to  make  a  rough  separation  of  the  finer 
particles,  by  pestling  up  10  grams  of  the  soil  with  water, 
and  pouring  off  the  supernatant  liquid  after  one  minute's 
standing,  as  in  a  mechanical  analysis.  Having  washed 
away  the  finer  portion  of  the  soil  two  or  three  times  in 
this  way,  the  residue  is  dried  and  the  carbonates  which 
remain  are  estimated  as  before,  thus  a  rough  idea  is 
obtained  of  their  distribution  among  the  finer  or  coarser 
sets  of  soil  particles. 

Interpretation  of  the  Results  of  a  Soil  Analysis. 

Though  much  may  doubtless  be  learnt  by  a  com- 
parison of  the  analysis  of  a  given  soil  with  the  analysis 
of  others  whose  fertility  has  been  proved  by  experience 
or  by  actual  manurial  experiments,  there  are  yet  many 
considerations  which  prevent  much  weight  being  attached 
to  the  results  thus  obtained. 

A  comparison  of  the  total  amount  of  any  of  the 
elements  of  plant  food  in  the  soil  with  the  amount  that 
is  withdrawn  by  an  ordinary  crop  shows  at  once  that 
even  in  the  poorest  soils  there  is  sufficient  material  for 
something  like  a  hundred  average  crops. 

The  density  of  the  surface  soil  has  already  been  dis- 
cussed ;  it  will  be  sufficiently  accurate  for  our  purpose 
if  we  consider  that  the  top  9  inches  of  one  acre  of 
an  ordinary  arable  field  weighs  2,500,000  Ibs.  On  this 
basis,  and  without  taking  into  account  the  fact  that 


152        THE  CHEMICAL  ANALYSIS  OF  SOILS      [CHAP. 

the  roots  of  most  cultivated  plants  range  far  deeper  than 
9  inches,  there  is  yet  present  about  2500  Ibs.  per  acre 
of  nitrogen,  potash,  and  phosphoric  acid  in  a  soil  con- 
taining only  o-i  per  cent,  of  these  constituents,  which 
is  about  the  lower  limit  usually  found.  The  following 
table  shows  the  amounts  of  these  food  materials — 
nitrogen,  phosphoric  acid,  and  potash — which  are  taken 
from  the  soil  by  an  average  crop  grown  in  rotation. 


Lbs.  per  acre. 

Wheat. 

Swedes. 

Barley. 

Clover. 

Nitrogen 

41-7 

940 

49-0 

IS9-3* 

Phosphoric  Acid    . 

20-5 

24-1 

20-7 

28-2 

Potash  . 

3-56 

93-5 

357 

102-4 

*  Partly  derived  from  the  atmosphere. 

It  is  clear  from  a  comparison  of  this  table  with  the 
quantities  previously  specified,  that  even  the  poorest  soil 
contains  the  nutrient  material  required  by  any  ordinary 
crop  many  times  over,  yet  we  know  that  crops  respond 
vigorously  to  dressings  of  manure  which  only  add  a 
fraction  to  the  plant  food  already  stored  in  the  soil. 
For  example,  a  wheat  crop  on  poor  soil  would  often  be 
doubled  by  the  use  of  2  cwt.  of  nitrate  of  soda  per  acre, 
/>.,  by  the  addition  of  35  Ibs.  of  nitrogen  in  nitrate  of 
soda  to  a  soil  that  already  contained  in  the  top  9  inches 
more  than  2000  Ibs.  per  acre.  Again,  4  cwt.  per  acre 
of  superphosphate,  containing  about  60  Ibs.  of  phosphoric 
acid,  will  be  necessary  in  the  usual  rotation  to  secure 
a  good  swede  crop,  though  there  may  be  already  2000 
to  3000  Ibs.  of  phosphoric  acid  in  the  soil.  We  are 
then  driven  to  conclude  that  the  nitrogen,  potash,  and 
phosphoric  acid  are  present  in  the  soil  in  some  other 
mode  of  combination  than  the  form  in  which  they  exist 
in  manures :  so  that  although  they  may  be  in  the  soil 


vi.]     RESERVES  OF  PLANT  FOOD  IN  THE  SOIL     153 

they  are  in  such  a  state  as  to  be  very  partially  of  service 
to  the  growing  plant.  Further  evidence  of  the  enormous 
stores  of  plant  food  in  the  soil  and  the  comparative  slow- 
ness with  which  they  can  be  utilised  may  be  obtained 
by  considering  the  results  obtained  at  Rothamsted, 
where  on  one  plot  wheat  has  been  grown  continuously 
without  manure  for  sixty-four  years  (to  1907).  The 
average  yield  from  this  plot  was  for  the  first  twenty  years, 
1844-63,  16-3  bushels  of  grain  and  15-1  cwt.  of  straw; 
1 1 -6  bushels  of  grain  and  9-3  cwt.  of  straw  for  the  next 
twenty  years,  1864-83;  and  12-3  bushels  of  grain  and 
8-7  cwt.  of  straw  for  the  third  period  of  twenty  years, 
1 884- 1 903.  It  is  calculated  that  during  the  last  fifty  years 
there  have  been  removed  from  this  plot  about  900  Ibs. 
per  acre  of  nitrogen,  470  of  phosphoric  acid,  and  760  of 
potash,  i.e.,  about  18,  9,  and  15  Ibs.  per  acre  per  annum 
respectively ;  yet  from  analyses  of  a  sample  taken  in  1 893 
the  surface  soil  to  a  depth  of  9  inches  still  contained 
o  1 1  per  cent,  of  nitrogen,  0-114  per  cent,  of  phosphoric 
acid,  and  0-38  per  cent,  of  potash  soluble  in  strong 
hydrochloric  acid,  or  2750,  2850,  and  9500  Ibs.  per 
acre  respectively.  The  soil  must  therefore  be  re- 
garded as  possessing  most  of  its  plant  food  in  states 
of  combination  that  cannot  be  utilised  by  the  plant, 
and  these  forms  slowly  pass,  by  weathering  and  other 
changes,  into  material  which  is  available  for  the  crop. 
The  plant  food  of  the  soil  represents  so  much  capital, 
and,  as  in  many  another  business,  but  a  small  proportion 
of  the  capital  is  liquid  at  any  given  time :  it  is  largely 
the  object  of  cultivation  to  effect  such  a  turnover  of  the 
capital  as  will  liquidate  some  of  it  in  a  form  available 
for  the  nutrition  of  the  crop. 

In  the  old  systems  of  agriculture,  before  the  land  was 
enclosed,  the  whole  crop  was  grown  out  of  capital, 
nothing  but  labour  was  put  into  the  soil :  in  which  con- 


154        THE  CHEMICAL  ANALYSIS  OF  SOILS      [CHAP. 

nection  it  is  interesting  to  note  that  the  original  mean- 
ing of  manure  was  to  work  by  hand.* 

It  becomes  important,  then,  to  attempt  to  discriminate 
between  the  various  forms  in  which  the  nitrogen, 
potash,  and  phosphoric  acid  may  be  present  in  the  soil, 
according  as  they  are  soluble,  or  likely  in  a  short  time 
to  become  sufficiently  soluble  to  reach  the  crop.  In  the 
case  of  nitrogen  we  know  that  of  the  various  compounds 
such  as  proteins  and  protein  residues,  amides,  ammonia 
salts,  and  nitrates  which  can  be  detected  in  the  soil,  only 
the  latter  can  enter  the  plant,  but  that,  by  processes  of 
fermentation,  all  of  the  other  compounds  will  eventually 
pass  into  the  state  of  nitrate.  Of  the  immediately  soluble 
nitrogen  compounds — nitrates,  nitrites,  and  ammonia — 
a  very  small  amount,  varying  from  5  to  200  Ibs.  per 
acre,  is  ever  present  in  the  soil  at  any  given  time, 
though  it  is  constantly  being  renewed  by  fermentation 
processes. 

Phosphoric  acid  also  exists  in  the  soil  in  many 
distinct  compounds  :  in  combination  with  carbon,  etc., 
it  is  found  in  nuclein  and  lecithin,  which  in  a  more 
or  less  humified  condition  are  found  among  the  plant 
and  animal  residues :  it  also  occurs  as  phosphate  of 
the  sesquioxides  of  iron  and  alumina ;  as  tribasic,  and 
probably  also  as  dibasic  phosphate  of  lime.  Of  these 
compounds  the  latter  are  undoubtedly  the  most  soluble 
in  either  pure  water  or  the  carbonic-acid-charged  water  of 
the  soil,  but  much  must  depend  on  the  physical  condition, 
as  well  as  on  the  chemical  combination,  in  which  the 
material  exists.  For  example,  when  using  tribasic  phos- 
phate of  lime  as  a  manure,  the  softer  phosphates,  such  as 
steamed  bone  flour,  are  more  effective  than  the  chemically 
similar  but  harder  material  in  ground  rock  phosphate. 

*  Cf.  Defoe,  Robinson  Crusoe  (1719) — "The  ground  that  I  had 
manured  or  dug  up  for  them  was  not  great." 


vi.]         DORMANT  AND  ACTIVE  PLANT  FOOD         155 

It  is  not  so  easy  to  classify  the  various  compounds 
of  potash  existing  in  the  soil :  we  know  that  as  felspar 
passes  into  kaolinitc  there  are  intermediate  stages  of 
weathering  in  which  the  potash  is  gradually  becoming 
more  soluble  in  soil  water,  but  it  is  impossible  to  isolate 
or  classify  the  various  hydrated  silicates  containing 
potash  that  must  exist.  Potash,  again,  which  has  once 
been  dissolved,  is  caught  and  retained  by  the  soil  in 
various  ill-defined  compounds,  some  of  which  must 
reach  the  crop  more  rapidly  than  others. 

The  work,  then,  of  soil  analysis  must  be  extended 
to  include  some  investigation  of  the  condition  of  the 
plant  food  in  the  soil,  as  well  as  its  absolute  quantity : 
it  is  not  enough  to  determine  what  constituents  are 
present  with  the  view  of  making  good  the  deficiencies, 
because  there  is  always  more  than  enough  for  many 
crops ;  inquiry  must  be  rather  directed  towards  finding 
how  much  is  likely  to  reach  the  crop.  The  attempt 
to  discriminate  between  the  total  and  what  may 
be  termed  the  available  plant  food  in  the  soil,  i.e., 
that  which  is  in  a  form  the  crop  can  immediately 
utilise,  has  been  made  in  two  ways  —  by  using  the 
growing  plant  as  an  analytical  agent,  or  by  attacking 
the  soil  with  very  dilute  acids,  whose  action  is  akin 
to  the  natural  solvent  agencies  at  work  when  the 
plant  is  growing.  The  former  process  proceeds  upon 
the  assumption  that  any  given  plant  has  a  certain 
average  composition  which  it  will  acquire  when  freely 
supplied  with  all  the  elements  of  nutrition ;  if  this 
plant  be  grown  upon  a  soil  deficient  in  one  particular, 
that  deficiency  will  be  reflected  in  the  analysis  of  the 
plant  when  fully  grown.  It  is  thus  necessary  to  select 
a  standard  plant  and  grow  it  under  normal  conditions 
of  manuring  to  ascertain  the  proportion  that  nitrogen, 
phosphoric  acid,  and  potash  usually  bear  to  the  ash. 


156        THE  CHEMICAL  ANALYSIS  OF  SOILS      [CHAP. 

The  selected  plant  is  then  grown  upon  the  soil  in 
question,  gathered  at  the  appropriate  stage  and  ana- 
lysed, when  the  composition  of  the  ash,  as  compared 
with  its  composition  under  normal  conditions,  should 
give  indications  of  the  state  of  the  soil.  Various  dis- 
turbing factors  come  into  play ;  for  example,  the  presence 
in  the  soil  of  large  quantities  of  a  non-essential  material 
like  calcium  sulphate  or  sodium  chloride  lowers  the 
proportion  that  potash  bears  to  the  total  ash  without 
necessarily  indicating  any  want  of  potash ;  again,  a 
deficiency  of  nitrogen  is  more  seen  in  a  general  stunting 
of  the  whole  development  of  the  plant  than  in  a  com- 
parative poverty  of  nitrogen  in  the  final  growth.  But 
by  selecting  suitable  test  plants,  valuable  indications  can 
be  obtained  as  to  the  need  or  otherwise  for  specific 
manuring.  As  a  rule,  cereals  are  unsuitable  test 
plants,  since  they  are  well  able  to  satisfy  their  require- 
ments for  mineral  nutrients  from  comparatively  im- 
poverished soils ;  the  straw  of  barley,  however,  shows 
considerable  variations  from  which  the  condition  of  the 
soil  as  regards  its  supply  of  phosphoric  acid  and  potash 
can  be  interpreted.  The  phosphoric  acid  in  the  ash  of 
barley  straw  will  vary  between  2  and  4  per  cent,  and 
the  potash  between  6  and  24  per  cent,  and  as  the 
straw  of  barley  grown  without  special  manuring  can 
readily  be  obtained,  it  forms  a  convenient  test  plant. 
The  most  sensitive  test  plants  are  provided  by  roots — 
swedes  for  estimating  the  phosphoric  acid,  and  mangolds 
for  estimating  the  potash  in  the  soils  on  which  they 
have  been  grown.  The  phosphoric  acid  in  the  ash  of 
swedes  has  been  found  as  low  as  9  per  cent,  when  the  soil 
was  one  that  responded  readily  to  phosphatic  manures, 
rising  to  16  per  cent,  when  the  soil  was  one  that  required 
no  phosphatic  manure.  Similarly,  the  potash  in  the  ash 
of  mangolds  will  vary  between  1 2  and  40  per  cent. 


vi.]        ENTRY  OF  PLANT  FOODS  BY  OSMOSIS        157 

The  method  which  is  now  very  largely  employed 
to  determine  the  mineral  plant  food  in  the  soil  that 
may  be  regarded  as  immediately  "  available "  for  the 
crop,  consists  in  attacking  the  soil  with  a  very  dilute 
acid,  whose  action  shall  be  comparable  with  the  natural 
solution  processes  bringing  nutriment  to  the  plant.  The 
mineral  matter  finds  it  way  by  osmosis  into  the  plant 
in  two  ways  :  either  from  the  natural  soil  water,  or  from 
the  more  concentrated  solution  formed  in  immediate 
proximity  to  the  root-hairs  by  the  attack  of  the  excreted 
carbon  dioxide  upon  the  soil  particles. 

The  natural  soil  water  is  constantly  dissolving 
small  quantities  of  phosphoric  acid,  potash,  and  other 
materials,  in  which  it  is  aided  by  the  carbonic  acid 
it  also  contains ;  as  this  water  passes  by  osmosis  into 
the  root-hairs  it  will  carry  with  it  the  dissolved 
material,  with  the  exception  of  any  particular  ion  or 
radicle  which  has  already  attained  in  the  cell  sap  a 
higher  concentration  than  it  possesses  in  the  external 
soil  solution.  But  if  the  soil  water  alone  brought  the 
mineral  matter  with  it,  not  enough  enters  the  plant 
to  account  for  the  observed  facts.  For  example,  the 
growth  of  a  crop  of  a  ton  and  a  half  of  clover  hay 
requires  the  transpiration  through  the  leaves,  and 
therefore  the  absorption  at  the  root,  of  about  400 
tons  of  water  (see  p.  91) ;  the  same  crop  would  also  contain 
about  50  Ibs.  of  potash.  If,  then,  the  crop  derived  all 
its  mineral  matter  by  the  simple  inflow  of  the  soil 
water  into  the  root,  the  50  Ibs.  of  potash  must  have 
been  originally  dissolved  in  the  400  tons  of  water 
that  passed  through  the  crop,  which  means  that  the 
soil  water  contained  as  much  as  0-006  per  cent,  of 
potash,  a  greater  concentration  than  is  observed  in 
humid  climates.  In  fact,  the  particular  ions  or  radicles 
concerned  in  nutrition  enter  the  root  faster  than  the 


158        THE  CHEMICAL  ANALYSIS  OF  SOILS      [CHAP. 

water  does ;  they  diffuse  through  the  cell  wall  because 
the  sap  within  is  maintained  in  a  less  concentrated 
state  as  far  as  they  are  concerned  than  the  external 
soil  water,  because  they  are  constantly  being  withdrawn 
from  solution  by  the  living  protoplasm  of  the  cells. 

It  has  been  supposed  that  solvent  action  of  the  soil 
water  is  also  assisted  by  the  cell  sap  of  the  root-hairs,  which 
is  always  distinctly  acid  in  its  reaction ;  these  root-hairs 
are  always  very  closely  in  contact  with  soil  particles, 
and  some  of  the  acid  has  been  supposed  to  diffuse  out- 
wards through  the  cell  wall.  Sachs  has  shown  that  a 
polished  slab  of  marble  is  etched  wherever  the  fine 
roots  of  a  plant  came  in  contact  with  it,  and  on  the 
strength  of  this  and  similar  experiments,  the  cell  sap 
has  been  regarded  as  a  factor  in  bringing  the  minerals 
of  the  soil  into  solution  for  the  plant.  All  the  solvent 
actions,  however  attributed  to  the  cell  sap,  can  be 
brought  about  by  the  carbon  dioxide  which  is  always 
being  excreted  by  the  root,  and  more  critical  experiments 
seem  to  negative  the  opinion  that  any  fixed  acids  pass 
outwards  through  the  cell  wall  of  a  living  plant,  at  any- 
rate  after  it  has  passed  the  seedling  stage. 

Whatever  the  theories  which  have  been  formed  as 
to  the  manner  in  which  the  mineral  constituents  of  the 
soil  pass  into  solution  for  the  plant,  it  is  improbable  that 
the  conditions  can  be  reproduced  in  the  laboratory,  and 
for  the  practical  purposes  of  analysis  the  desideratum  is 
a  solvent  that  will  dissolve  the  class  of  material  which 
is  found  by  experience  to  reach  the  immediate  crop,  but 
which  will  not  touch  the  same  material  should  its  state 
of  combination  or  physical  condition  be  such  as  to  render 
it  unavailable  for  the  plant.  Various  solvents  have  been 
proposed :  for  example,  Deherain  showed  that  dilute 
acetic  acid,  while  dissolving  some  phosphoric  acid  from 
ordinary  soils,  was  incapable  of  extracting  any  from  a 


vi.]    SOLVENTS  FOR  AVAILABLE  PLANT  FOOD     159 

particular  soil  which  yielded  very  poor  crops  unless  man- 
ured with  superphosphate,  though  it  contained  o-i  per 
cent,  of  phosphoric  acid  soluble  in  strong  hydrochloric 
acid.  Hence  he  concluded  that  dilute  acetic  acid 
forms  a  solvent  only  capable  of  attacking  the  avail- 
able phosphoric  acid.  A  solution  of  carbonic  acid  has 
been  suggested  as  akin  to  the  natural  soil  water ; 
other  solutions  have  been  employed  because  they  will 
dissolve  certain  of  the  compounds  of  phosphoric  acid 
in  the  soil,  but  not  all — the  calcium  phosphates,  for 
example,  but  not  the  phosphates  of  iron  and  alumi- 
nium ;  other  solvents,  again,  are  recommended  as 
akin  to  the  acid  cell  sap.  However,  experience  seems 
to  show  that  the  i  per  cent,  solution  of  citric  acid  pro- 
posed by  Dyer  in  1894  gives  results  that  are  most  in 
accord  with  what  is  known  of  the  soil,  either  from  its 
past  history  or  by  cropping  experiments. 

The  method  of  conducting  the  analysis  is  as  follows : 
—200  grams  of  the  "  fine  earth  "  that  has  passed  the  3 
mm.  sieve,  in  its  air-dried  state,  is  placed  without  any 
further  grinding  in  a  dry  Winchester  quart  bottle  with 
20  grams  of  pure  crystallised  citric  acid  and  2  litres  of 
water.  The  bottle  should  either  be  one  previously  used 
for  the  storage  of  strong  acids  or  should  have  a  pre- 
liminary soaking  in  dilute  hydrochloric  acid.  The 
mixture  of  soil  and  dilute  acid  is  thoroughly  shaken  from 
time  to  time,  as  often  as  may  be  convenient,  during  the 
seven  days  the  solvent  action  is  allowed  to  proceed. 
After  seven  days  the  solution  is  filtered,  and  two  aliquot 
portions  of  500  c.c.  each  are  evaporated  to  dryness  and 
ignited  to  get  rid  of  the  citric  acid  and  other  organic 
matter.  The  residues  are  dissolved  in  hydrochloric  acid, 
again  evaporated,  and  heated  for  a  time  to  105  C.  to 
render  all  the  silica  insoluble.  In  one  portion  the 
phosphoric  acid,  and  in  the  other  the  potash,  are  deter- 


160          THE  CHEMICAL  ANALYSIS  OF  SOILS    [CHAP. 

mined  by  the  processes  previously  described.  The  time 
of  extraction  may  be  shortened  to  twenty-four  hours  if 
the  bottle  be  put  in  a  good  end-over-end  shaking 
machine  which  will  keep  the  soil  and  the  solvent 
thoroughly  agitated. 

An  examination  of  the  citric  acid  solution  shows  that 
all  the  compounds  of  phosphoric  acid  that  have  been 
indicated  as  existing  in  the  soil  are  more  or  less 
attacked;  at  any  rate,  the  resulting  solution  contains 
organic  matter  and  salts  of  aluminium  and  iron,  in 
addition  to  calcium.  It  has  been  suggested  that  the 
varying  amounts  of  calcium  carbonate  contained  by  soils 
will  much  affect  the  material  dissolved  by  the  citric  acid, 
some  of  which  becomes  neutralised  by  the  calcium 
carbonate.  But  though  the  amount  of  phosphoric  acid 
dissolved  from  a  given  soil  by  the  citric  acid  solution  will 
be  diminished  if  the  calcium  carbonate  in  the  soil  is 
increased,  a  very  similar  reduction  will  be  effected  in  the 
natural  processes  of  solution  of  the  soil  phosphates  under 
field  conditions.  No  attempt  should  be  made  to  add  an 
extra  amount  of  citric  acid  to  combine  with  the  calcium 
carbonate  ;  secondary  solvent  actions  are  set  up  both  by 
the  carbon  dioxide  evolved  and  by  the  calcium  citrate 
formed,  moreover,  the  real  comparative  basis  of  the 
method  of  analysis  is  destroyed. 

It  must  not  be  supposed  that  the  citric  acid  solution, 
nor  indeed  any  of  dilute  acid  solvents  that  have  been 
proposed  for  this  purpose,  are  real  differential  solvents, 
which  extract  the  material  in  the  soil  which  is  available 
for  the  plant  and  leave  untouched  whatever  is  combined 
in  some  other  form.  In  reality,  as  soon  as  the  acid  has 
been  for  a  sufficient  time  in  contact  with  the  soil  a  state 
of  equilibrium  is  attained  between  the  phosphoric  acid, 
for  example,  that  has  gone  into  solution  and  that  which 
remains  in  the  solid  state.  The  precise  equilibrium 


vi.]  USE  OF  WEAK  ACID  SOLVENTS  161 

attained  depends  not  only  upon  the  strength  of  the  acid 
solution  and  the  nature  and  amount  of  the  phosphoric 
acid  compounds  in  the  soil,  but  also  on  the  nature  and 
amount  of  the  bases   that  are  there   present.     If,  for 
example,  the  citric  acid  solution  is  filtered  off  after  it  has 
extracted   all  the  phosphoric  acid  it  can,  and  a  second 
portion  of  solution  is  added  and  the  soil  extracted  afresh, 
then   more   phosphoric   acid   will   go  into  solution,  the 
amount  being  smaller  than  before  but  still  considerable. 
A  third,  a  fourth,  and  even  a  fifth  extraction  does  not 
remove  from   the  soil  all  the  phosphoric  acid  that  will 
go  into  solution  in  the  dilute  citric  acid  solution.     Thus 
it  is  impossible  to  say  that  the  dilute  citric  or  any  other 
acid  dissolves  out  and  measures  the  "available"   phos- 
phoric acid  or  potash ;  it  does,  however,  provide  a  figure 
indicating  the  comparative  rate  at  which  the  soil  is  likely 
to  yield  up  its  nutrient  constituents  to  the  normal  solvent 
actions  going  on  in  the  soil.     The  results,  then,  of  this 
method  of  analysis  are  not  to  be  regarded  as  absolute 
amounts,  but  as  empirically  obtained  figures  which  must 
be  interpreted  in  the  light  of  experience.     The  type  of 
the  soil  plays  a  part ;  for  example,  a  quantity  of  citric  acid, 
soluble  phosphoric  acid  that  would  indicate  poverty  in  a 
strong  loam  or  in  a  soil  rich  in  organic  matter  like  an 
old  pasture,  would  be  ample  for  ordinary  crop  purposes 
if  the  soil  were  light  and  sandy.     Again,  the  crop  must 
be  taken  into  account ;  a  percentage  indicating  enough 
available  phosphoric  acid  in  the  soil  for  wheat  or  man- 
golds  would  indicate   deficiency  when   the   swede  crop 
came  round. 

In  certain  cases,  by  continuing  the  extraction  with 
citric  acid  until  the  amount  going  into  solution  at  each  ex- 
traction becomes  approximately  constant  at  some  low 
figure,  it  is  possible  to  differentiate  between  the  phos- 
phates in  the  soil  that  are  easily  soluble  and  may 

L 


162         THE  CHEMICAL  ANALYSIS  OF  SOILS    [CHAP. 


therefore  be  termed  "  available,"  because  they  possess  a 
comparatively  high  solubility  factor,  and  other  phosphates 
which  would  yield,  under  natural  conditions,  solutions 
too  dilute  to  nourish  the  crop  efficiently. 

For  example,  the  following  table  shows  the  amounts 
of  phosphoric  acid  dissolved  by  successive  extractions  of 
certain  Rothamsted  soils  with  i  per  cent,  citric  acid 
solution,  from  which  it  will  be  seen  that  at  about  the  fifth 
extraction  the  quantity  dissolved  begins  to  approach  a 
constant. 

PHOSPHORIC  ACID,  MGMS.  PER  100  GRAMS  SOIL. 


EXTRACTION. 

1st. 

2nd. 

3rd. 

4th. 

6th. 

6th. 

Broadbalk 

3 

Unmanured 

6-4 

6-8 

3-9 

3-0 

2-5 

5 

64  Ibs.  P2O5,  no  Nitrogen 

69-0 

28-0 

II-3 

7-3 

4-5 

2-3 

7 

64  Ibs.  P2O5)  86  Ibs.  N.  . 

56-1 

22-8 

8-9 

6-5 

4-4 

4.4 

8 

64  lbs.P2O8,  129  Ibs.  N.  . 

4&-3 

18-9 

7-8 

5-3 

40 

30 

10 

86  Ibs.  N.  only 

7-7 

5-2 

3-3 

2-7 

2-7 

2.7 

2 

Dunged  .... 

49-3 

15-3 

7-5 

6-0 

4-4 

Hoos 

lAA 

43  Ibs.  N.  only 

6-3 

3-5 

2-2 

1-9 

2-0 

1-2 

ii 
it 

2AA 
3AA 

64  Ibs.  P2O6,  43  Ibs.  N.  . 
43  Ibs.  N.  and  Potash      . 

52-2 
6-3 

21-2 

2-7 

8-9 

2-3 

6-5 

2-1 

3-8 

I-O, 

2.9 

i-5 

i) 

4AA 

64  Ibs.  P2O3,  43  Ibs.  N., 

Potash 

53-5 

10-6 

6.4 

4-9 

4-5 

3-8 

The  successive  amounts  going  into  solution  in  the 
first  four  or  five  extractions  of  the  soil  from  the  plots 
which  had  received  soluble  phosphoric  acid  every  year 
are  found  to  be  decreasing  in  a  logarithmic  series,  and 
this  may  be  supposed  to  indicate  that  the  solvent  is 
dealing  with  only  one  class  of  material,  which  is  entirely 
removed  at  about  the  fifth  extraction.  After  the 
fifth  extraction  there  only  remains  the  more  insoluble 
classes  of  phosphates  which  form  the  main  stock  in 


vi.] 


A  VAILABLE  PLANT  FOOD 


163 


the  soil.  The  unmanured  plot  and  that  which  has 
received  dung  do  not  show  the  same  regular  decrement, 
indicating  that  the  solvent  is  each  time  dealing  with  a 
more  complex  mixture  of  phosphates  successively  going 
into  solution.  This  conclusion  is  strengthened  when  the 
total  amount  of  phosphoric  acid  dissolved  in  five  extrac- 
tions is  compared  with  the  amount  known  to  have  been 
applied  to  the  land  during  the  period  the  plots  have 
been  under  experiment,  after  deduction  has  been  made 
of  that  which  is  also  known  to  have  been  removed  in  the 
crop. 

PHOSPHORIC  ACID  IN  ROTHAMSTED  SOILS. 


DISSOLVED  ix  KIVE 

EXTRACTIONS. 

Supplied 

Removed 

Lbs 

Manure. 

Crop. 

Per  cent. 

Broadbalk 

3 

0-0226 

$65 

O 

550 

-550 

ii 

5 

O-I2OI 

300O 

3960 

790 

3170 

>i 

7 

0-0987 

2470 

3810 

1370 

2440 

8 

0-0823 

2055 

3810 

1520* 

229O 

ti 

2 

0-0825 

2060 

4780 

1650 

3UO 

Hoos 

lAA 

0-0159 

400 

0 

555 

-555 

„ 

2AA 

0-0926 

2315 

3390 

1200 

2190 

it 

4AA 

00799 

2OOO 

3390 

I24O 

2150 

*  Approximate  estimate,  since  the  crop  has  rarely  been  analysed. 

It  will  be  seen  from  the  above  table  that  the  amount 
of  phosphoric  acid  dissolved  by  the  five  extractions 
agrees  closely  with  the  surplus  left  by  the  manuring  in 
all  the  cases  where  the  phosphoric  acid  has  been  put  on 
as  soluble  mineral  superphosphate.  This  is  not  the  case, 
however,  for  the  plot  manured  with  dung,  which  contains 
a  considerable  proportion  of  difficultly  soluble  phosphate. 

It  should  not  be  supposed  that  the  whole  of  the 
so-called  "  available "  phosphoric  acid  or  potash  will  be 


164          THE  CHEMICAL  ANALYSTS  OF  SOILS    [CHAP. 

removed  by  the  crop;  even  the  minimum  of  o-oi  per 
cent.,  soluble  in  citric  acid,  which  has  been  suggested, 
as  marking  the  limit  of  fertility,  means  about  250  Ibs. 
per  acre  in  the  surface  layer  9  inches  deep :  and  few 
crops  will  take  away  as  much  as  50  Ibs.  per' acre  of  phos- 
phoric acid  or  150  Ibs.  per  acre  of  potash.  No  crop 
searches  the  soil  so  thoroughly  for  food  as  does  the 
solvent  acid  :  if  we  assume  that  the  roots  themselves  by 
their  excretion  of  carbon  dioxide  effect  some  of  the 
solution,  it  is  obvious  that  they  come  in  contact  with  but 
a  small  proportion  of  the  soil  particles  ;  nor  can  the  soil 
water,  limited  in  amount  and  moving  slowly,  attack  the 
soil  with  the  vigour  displayed  by  an  acid  which  is 
continuously  shaken  with  a  comparatively  small  propor- 
tion of  soil.  Even  in  the  case  of  material  so  essentially 
"  available  "  as  a  manure  soluble  in  water,  the  whole  of 
the  manure  applied  is  never  recovered  in  the  crop ;  e.g., 
in  the  experiments  with  wheat  at  Rothamsted,  only  73 
per  cent,  and  with  mangolds  78  per  cent.,  of  the  nitrogen 
supplied  as  nitrate  of  soda  has  been  recovered  in  the 
crop,  though  there  was  an  abundant  supply  of  the  other 
manurial  constituents.  In  the  same  way,  on  the  plot 
with  an  excess  of  nitrogen  there  was  recovered  only 
36  per  cent,  of  the  phosphoric  acid  supplied  as  super- 
phosphate, and  50  per  cent,  of  potash  supplied  as  sulphate 
of  potash. 

In  other  words,  the  "available"  plant  food  in  the 
soil  represents  not  that  which  the  succeeding  crop  will 
remove,  but  that  which  it  can  draw  upon  :  how  much 
it  will  acquire  will  depend  on  a  variety  of  factors,  such 
as  the  nature  of  the  plant,  the  texture  of  the  soil, 
the  supply  of  water,  and  other  necessaries  of  nutrition. 

No  method  akin  to  solution  in  dilute  citric  acid 
has  yet  been  devised  for  determining  what  proportion 
of  the  nitrogen  reserves  in  the  soil  is  likely  to  be  avail- 


vi.]       DETERMINATION  OF  SOLUBLE  HUMUS        165 

able.  The  conversion  of  the  nitrogenous  matter  of 
the  soil  into  soluble  nitrates,  in  which  form  nitrogen 
enters  the  plant,  is  a  biological  process  which  is  influ- 
enced by  a  number  of  conditions,  such  as  temperature, 
degree  of  moisture  and  aeration  of  the  soil,  the  mechani- 
cal treatment  it  receives,  all  impossible  to  predict. 

Some  idea  of  the  condition  of  the  organic  matter 
and  the  readiness  with  which  it  is  likely  to  change,  may 
be  obtained  by  a  determination  of  the  humus  soluble  in 
dilute  ammonia  and  the  percentage  of  nitrogen  in  this 
humus,  or  again  by  a  study  of  the  ratio  of  carbon  to 
nitrogen  in  the  organic  matter  as  previously  indicated 
(p.  46).  In  order  to  determine  the  soluble  humus,  20 
grams  of  the  soil  are  digested  with  enough  i  per  cent, 
hydrochloric  acid  to  dissolve  all  the  calcium  carbonate, 
thrown  upon  a  filter,  washed  with  a  little  more  of  the 
hydrochloric  acid  and  then  with  water  until  neutral. 
The  soil  is  then  washed  off  the  filter  into  a  bottle  with  a 
4  per  cent,  solution  of  ammonia  and  shaken  for  twenty- 
four  hours,  after  which  the  bottle  is  left  to  stand  until 
the  bulk  of  the  inorganic  matter  of  the  soil  has  settled. 
1 50  c.c.  are  pipetted  off  and  evaporated  to  dryness  over 
the  water-bath,  weighed  and  ignited,  a  deduction  being 
made  of  the  inorganic  matter  remaining  after  ignition. 

To  determine  the  nitrogen  content  of  the  humus  a 
second  1 50  c.c.  are  placed  in  a  Kjeldahl  flask.  Two  or 
three  grams  of  magnesia  are  added  and  the  \vhole 
evaporated  to  dryness  to  get  rid  of  the  ammonia  in  the 
solution,  after  which  the  contents  of  the  flask  are 
digested  with  sulphuric  acid  in  the  usual  manner. 

Valuable  as  these  determinations  may  become  in 
judging  a  soil,  a  sufficient  body  of  data  do  not  as  yet 
exist  to  enable  them  to  be  interpreted  with  precision. 

In  the  analysis  of  a  soil,  without  doubt  the  most 
important  figure  is  the  proportion  of  calcium  carbonate, 


1 66          THE  CHEMICAL  ANALYSIS  Of  SOILS    [CHAP. 

for  on  that  must  be  based  the  decision  not  only  of 
whether  liming  is  necessary,  but  what  class  of  artificial 
manures  should  be  employed.  Where  the  calcium 
carbonate  is  scanty,  manures  like  superphosphate  and 
sulphate  of  ammonia  should  never  be  employed,  but  basic 
slag  or  some  neutral  phosphate  on  the  one  hand,  and 
nitrate  of  soda  as  a  source  of  rapidly  acting  nitrogen  on 
the  other.  The  texture  of  the  soil,  the  rapidity  with 
which  decay  and  nitrification  of  organic  matter  take 
place,  freedom  from  fungoid  diseases,  all  depend  on  an 
adequate  proportion  of  calcium  carbonate  in  the  soil, 
say  from  half  to  one  per  cent. ;  so  that  of  all  the 
determinations  this  is  the  most  important. 

The  determinations  of  the  loss  on  ignition,  the 
nitrogen,  and  possibly  the  humus,  give  the  analyst  an 
idea  of  the  reserves  of  organic  matter  in  the  soil ; 
judged  in  conjunction  with  the  mechanical  analysis  and 
the  proportion  of  calcium  carbonate,  an  opinion  can  be 
formed  as  to  the  condition  of  the  soil  and  how  far  these 
reserves  are  likely  to  be  brought  into  play  by  cultiva- 
tion. An  opinion  may,  again,  be  formed  as  to  the  need 
for  organic  manures  to  increase  the  humus  content  of 
the  soil,  or  whether  fertility  is  likely  to  be  maintained 
with  purely  mineral  manures. 

A  consideration  of  the  available  phosphoric  acid  and 
potash  will  give  the  analyst  an  idea  of  the  immediate 
need  or  otherwise  of  mineral  manuring ;  the  proportions 
these  bear  to  the  "total"  phosphoric  acid  and  potash 
give  him  grounds  for  deciding  whether  the  lack  is  only 
temporary  or  real.  In  the  former  case  measures  may 
be  taken  to  liberate  some  of  the  reserves,  as  by  the 
judicious  use  of  lime  or  of  organic  manures  which  will 
generate  carbonic  and  other  acids  within  the  soil. 

Such  further  questions  as  the  presence  of  harmful 
substances,  or  even  of  an  excess  of  more  normal  con- 


VI.]  INTERPRETATION  OF  ANALYSES  167 

stituents  of  the  soil,  must  be  considered  by  the  analyst, 
but  will  be  dealt  with  in  a  later  section. 

In  some  cases  it  will  be  possible  by  a  chemical 
analysis  to  pronounce  a  given  soil  to  be  unsuited  to  a 
particular  crop :  as  a  rule,  however,  it  is  not  its  chemical 
composition  which  fits  the  land  for  a  particular  crop, 
but  its  mechanical  texture,  water-bearing  power,  drain- 
age, etc.  In  most  cases  the  soil  can  be  adjusted  to  the 
crop  by  manure,  though  the  process  may  be  unsound 
from  an  economic  standpoint,  but  no  expenditure  can 
ever  rectify  unsatisfactory  texture,  e.g.,  convert  a  light 
sand  into  good  wheat  land. 

Even  in  considering  the  chemical  analysis  of  a 
soil,  no  hard-and-fast  rules  can  be  laid  down,  the 
judgment  and  experience  of  the  analyst  must  come  into 
play  in  deciding  how  far  the  deficiency  or  excess  of  one 
constituent  is  likely  to  affect  the  action  of  some  of  the 
others :  and  again,  how  far  the  texture,  the  aspect,  and 
other  factors  that  can  only  be  ascertained  in  situ,  will 
exercise  an  influence  upon  the  enormous  reserves  of 
plant  food  contained  in  every  soil. 


CHAPTER  VII 

THE  LIVING  ORGANISMS  OF  THE  SOIL 

Decay  and  Humification  of  Organic  Matter  in  the  Soil — Alinit — 
The  Fixation  or  Free  Nitrogen  by  Bacteria  living  in  Sym- 
biosis with  Leguminous  Plants — Soil  Inoculation  with  Nodule 
Organisms — Fixation  of  Nitrogen  by  Bacteria  living  free  in  the 
Soil  —  Nitrification  —  Denitrification  — Iron  Bacteria  —  Fungi 
of  Importance  in  the  Soil :  Mycorhiza,  and  the  Slime  Fungus 
of  "  Finger-and-Toe." 

THE  soil  is  the  seat  of  a  number  of  slow  chemical 
changes  affecting  the  organic  material  it  receives : 
residues  of  an  animal  or  vegetable  nature,  when  applied 
to  the  soil,  are  converted  into  the  dark-coloured  complex 
known  as  "  humus,"  which  becomes  eventually  oxidised 
to  carbonic  acid,  water,  nitric  acid,  and  other  simple 
substances  serving  as  food  for  plants.  These  changes, 
at  one  time  regarded  as  purely  chemical,  are  now 
recognised  as  dependent  upon  the  vital  processes 
of  certain  minute  organisms,  universally  distributed 
throughout  cultivated  soil,  and  subject  to  the  same 
laws  of  nutrition,  multiplication,  life  and  death,  as 
hold  for  the  higher  organisms  with  which  we  are  more 
generally  familiar. 

The  microscopic   flora   of  the   soil,  roughly  classed 
as  fungi  and  bacteria,  is  vast,  and  has  been  very  in- 
adequately explored  as   yet :    certain  types  of  change 
ics 


CHAP,  vii.]      TYPES  OF  BACTERIAL  ACTION  169 

in  the  soil  materials  have,  however,  been  associated 
with  particular  organisms  or  groups  of  organisms,  and 
many  of  these  changes  are  of  fundamental  importance 
in  the  ordinary  nutrition  of  plants.  The  organisms  in 
the  soil  which  so  far  have  received  the  chief  attention 
are  those  concerned  with  the  supply  of  nitrogen  to  the 
plant.  Certain  organic  compounds  of  nitrogen,  chiefly 
of  a  protein  nature,  become  gradually  broken  down  by 
the  action  of  soil  bacteria  into  simpler  compounds,  e.g., 
into  amino-acids,  and  then  into  ammonia,  which  latter 
substance  is  seized  upon  by  other  organisms  and  oxidised 
successively  to  nitrous  and  nitric  acid.  As  nitric  acid 
is  almost  the  only  form  in  which  the  higher  plants 
obtain  the  nitrogen  they  require,  the  fertility  of  the 
soil  is  wholly  bound  up  in  the  maintenance  of  this 
cycle  of  change.  Under  certain  conditions  the  work 
of  other  organisms  intervenes,  and  the  nitrogen  com- 
pounds, instead  of  becoming  nitric  acid,  are  converted 
into  free  nitrogen  gas,  and  are  lost  to  the  soil.  Per 
contra,  another  group  of  organisms  possesses  the  power 
of  "fixing"  free  nitrogen,  />.,  of  taking  the  gaseous 
element  nitrogen  and  combining  it  with  carbon, 
hydrogen,  oxygen,  etc.,  into  forms  available  for  the 
higher  plants.  Such  organisms  sometimes  act  when 
living  in  "symbiosis"  with  plants  possessing  green 
carbon-assimilating  tissue :  the  two  form  a  kind  of 
association  for  mutual  support,  the  bacteria  deriving 
the  carbohydrate  which  they  must  consume  from  the 
higher  plant  supplied  by  them  with  combined  nitrogen. 

Other  symbiotic  processes  have  been  traced  in  the 
soil,  and  may  yet  be  made  to  play  an  important  part 
in  the  nutrition  of  field  crops.  Indeed,  a  number  of 
tentative  trials  have  already  been  made  with  the  view 
of  increasing  the  productiveness  of  the  soil  by  introduc- 
ing either  useful  organisms  that  were  wanting,  or 


170     THE  LIVING  ORGANISMS  OF  THE  SOIL     [CHAP. 

improved  types  to  replace  already   existing  kinds  of 
less  effective  character. 


The  Changes  of  Organic  Matter  in  the  Soil. 

The  surface  layer  of  soil  is  constantly  receiving 
additions  of  organic  matter,  either  leaves  and  other 
debris  of  vegetation  covering  the  ground,  together  with 
the  droppings  of  animals  consuming  that  vegetation, 
or  dung  and  other  animal  and  vegetable  residues  which 
are  supplied  as  manures  to  cultivated  land.  These 
materials  rapidly  change  in  ordinary  soil,  losing  almost 
immediately  any  structure  they  possess,  becoming 
dark-coloured  humic  bodies,  or  even  burning  away  as 
thoroughly  as  if  placed  in  a  furnace.  That  these 
changes  are  due  to  micro-organisms  is  seen  by  their 
immediate  cessation  if  the  soil  be  treated  with  anti- 
septics like  chloroform  or  mercuric  chloride :  or  if  the 
mixture  of  soil  and  organic  matter  be  sterilised  by  heat- 
ing. Attempts  have  been  made  to  estimate  the  number 
of  bacteria  contained  in  the  soil :  the  prodigious  numbers 
obtained,  2  up  to  50  millions  or  more  per  cubic  centi- 
metre of  the  upper  soil,  show  little  beyond  the  fact  that 
the  soil  is  tenanted  much  as  any  other  decaying  organic 
material  would  be.  The  soil  bacteria  are  always  associ- 
ated with  a  certain  number  of  fungi  and  yeasts, 
especially  when  the  reaction  of  the  medium  is  at  all 
acid :  the  organisms  are  most  numerous  in  the  surface 
layer,  though  they  are  still  to  be  found  in  the  deepest 
subsoils.  Below  a  certain  depth  they  must  disappear, 
because  deep  well  water  often  comes  to  the  surface  in 
an  absolutely  sterile  condition.  The  changes  which 
organic  materials  undergo  in  the  soil  may  be  roughly 
grouped  into  two  classes ;  according  as  there  is  free 
access  of  oxygen  or  not,  either  decay  (eremacausis) 


vii.]  FORMATION  OF  HUMUS  171 

with  eventual  resolution  into  the  simplest  inorganic 
oxidised  compounds,  or  "  humification "  will  set  in. 
These  changes  can  be  best  indicated  by  the  fate  of  a 
dead  branch  when  it  falls  either  upon  the  ground,  or 
into  a  pond  or  swamp  where  it  becomes  buried  in  the 
mud  at  the  bottom.  In  the  latter  case  the  fermentation 
changes  cause  the  wood  to  darken  even  to  blackness ; 
gases  like  carbonic  acid  and  marsh  gas  are  split  off,  so 
that  the  material  becomes  proportionally  richer  in 
carbon  and  poorer  in  oxygen.  Eventually,  however,  the 
process  slackens,  the  losses  practically  cease,  and  a 
large  proportion  of  the  original  material  persists.  On 
the  other  hand,  the  branch  exposed  to  the  air,  without 
darkening  very  much,  becomes  slowly  resolved  by  the 
action  of  fungi  and  bacteria  into  carbonic  acid  and 
water,  ammonia,  nitrogen  gas,  and  mineral  salts,  with 
much  the  same  final  result  as  though  it  had  been  placed 
in  a  furnace.  In  soil,  both  these  types  of  change  may 
go  on,  and  the  conditions  of  the  soil  as  regards  aera- 
tion, drainage,  temperature,  and  cultivation,  determine 
which  will  predominate. 

Practically,  the  whole  group  of  aerobic  bacteria,  i.e., 
those  which  require  free  oxygen  for  their  development, 
and  fungi  are  capable  of  bringing  about  the  oxidation 
changes  which  result  in  the  production  of  carbonic 
acid,  the  combustion  of  some  carbohydrate  being 
essentially  the  means  by  which  they  derive  their 
energy.  As  an  intermediate  step  between  the  carbo- 
hydrate and  the  carbonic  acid,  a  certain  amount  of 
humus  is  produced — "  mould,"  or  the  "  mild  humus  " 
of  the  German  writers.  Examples  of  this  material 
can  be  seen  in  the  leaf-mould  collected  by  gardeners 
from  woods,  or  the  fine,  brown  powder  which  can  be 
scraped  out  of  the  inside  of  a  hollow  tree,  particularly 
of  a  willow;  this  mould  differs  from  the  peaty  humus, 


172      THE  LIVING  ORGANISMS  OF  THE  SOIL   [CHAP. 

to  be  described  later,  in  its  neutral  reaction  and  in 
the  readiness  with  which  it  can  be  further  oxidised. 
Neutral  in  its  reaction,  it  yields  but  little  soluble 
"  humic  acid  "  to  the  attack  of  an  alkali. 

Besides  carbohydrates,  most  aerobic  bacteria  require 
some  carbon  compound  of  nitrogen,  and  will  begin  to 
break  down  protein  and  other  nitrogen-containing 
materials.  The  products  of  their  attack  are  succes- 
sively peptones,  bodies  like  leucin  and  tyrosin,  even- 
tually ammonia,  and  probably  free  nitrogen,  but  the 
ultimate  production  of  ammonia  is  perhaps  the  most 
characteristic  feature  of  the  aerobic  fermentation  of 
protein  bodies.  Other  amides  are  also  resolved  into 
ammonia,  of  which  a  characteristic  example  is  afforded 
by  the  change  of  urea  into  ammonium  carbonate. 
This  process  [which  is  one  of  hydrolysis,  not  of  oxi- 
dation, being  represented  in  the  gross  by  the  equation 
CO(NH2).2+2H2O  =  (NH4)2CO3]  is  brought  about  by 
more  than  one  organism,  universally  distributed  and 
abundant  in  such  places  as  stables  and  cattle  stalls. 
In  warm  weather  the  conversion  of  the  urea  of  the 
urine  into  ammonium  carbonate  is  very  rapid,  and  as 
the  resulting  product  dissociates  into  gaseous  ammonia 
and  carbonic  acid,  to  this  cause  is  due  the  smell  of 
ammonia  which  is  always  to  be  noticed  in  such  places. 
These  changes  to  ammonia  are  the  necessary  prelim- 
inaries to  the  final  oxidation  process  or  nitrification, 
which,  as  the  means  by  which  the  higher  plants  receive 
their  supplies  of  nitrogen,  will  be  discussed  separately. 
The  various  oxidation  processes  in  the  soil  are,  like 
all  other  bacterial  actions,  promoted  by  a  certain 
warmth,  the  optimum  temperature  being  about  25"-3o°, 
by  a  sufficiency  of  moisture,  and  by  the  presence  of 
mineral  food,  like  phosphates  and  potash  salts.  In 
any  great  quantity,  however,  salts  are  harmful, 


vii.]  DECAY  OF  ORGANIC  MATTER  173 

particularly  sodium  chloride ;  an  acid  reaction  also 
diminishes  considerably  the  rate  of  decay.  Speaking 
generally,  bacteria  do  not  thrive  as  soon  as  the 
medium  passes  the  neutral  point,  and  all  the  decay 
processes  must  be  carried  out  by  the  development  of 
fungi  when  the  medium  is  acid.  The  presence  of 
chalk,  or  any  form  of  carbonate  of  lime,  by  neutral- 
ising any  acids  as  fast  as  they  are  formed,  promotes 
the  destruction  of  organic  matter.  Wollny  has  also 
shown  that  calcium  humate  will  oxidise  much  more 
rapidly  than  uncombined  humic  acid  placed  under 
similar  conditions.  To  the  absence  of  carbonate  of 
lime  and  mineral  salts  generally,  may  be  ascribed  the 
tendency  of  humus  to  accumulate  and  persist  on  the 
very  light,  sandy  heaths,  where  the  soil  is  dry  and 
hot  in  summer,  and  also  well  aerated.  It  has  already 
been  indicated,  in  treating  of  humus,  that  the  various 
organic  compounds  of  nitrogen  show  very  different 
susceptibility  to  the  breaking-down  process  which  even- 
tually renders  the  nitrogen  available  for  the  crop — 
amongst  the  most  resistent  substances  being  the  nucleo- 
proteins  in  the  undigested  portions  of  food  which  form 
dung,  and  the  humus  residues  from  poor,  cropped-out 
land.  As  in  all  cases  much  of  the  nitrogen  of  both  soil 
and  manure  seems  to  pass  into  obstinately  persistent 
compounds  yielding  slowly,  if  at  all,  to  oxidation,  and 
hence  wasted  to  the  farmer,  an  attempt  has  been  made 
to  increase  the  preliminary  breaking  down  of  nitrogen 
compounds  in  the  soil  by  the  introduction  of  certain 
very  active  bacteria.  Stoklasa  has  shown  that  various 
organisms — B.  megatherium,  B.  fluorescens,  etc. — when 
seeded  into  soil  manured  with  bone-meal  or  similar 
materials,  increase  both  the  nitrogen  and  the  phosphoric 
acid  obtained  by  the  plant.  A  pure  cultivation  of  some 
such  organism,  B.  Ellenbachensis>  was  for  a  time  sold 


174     THE  LIVING  ORGANISMS  OF  THE  SOIL     [CHAP. 

commercially  under  the  name  of  alinit,  and  though 
the  power  of  fixing  nitrogen  was  claimed  for  it,  its 
chief  action  was  probably  such  as  was  described  above.  It 
has  been  found  to  cause  increased  crop  returns  on  peaty 
or  other  soils  rich  in  humus,  or  where  slow-acting 
nitrogenous  manures  have  been  applied. 

The    fermentation    which    goes   on   in    absence    of 
oxygen,  is  brought  about  by  a  large  number  of  bacteria, 
some  of  which  are  only  active  in  the  absence  of  oxygen, 
others  are  aerobic,  but  will  continue  their  work  when 
deprived  of  free  oxygen.    Carbohydrates  are  decomposed 
with  formation  of  carbonic  acid   and  other  gases  like 
hydrogen  and  marsh  gas,  butyric  and  other  fatty  acids, 
a  residue  of  humus  being  always  produced  at  the  same 
time.     The  protein  bodies  readily  undergo  putrefactive 
change,  with    the    production   of   tyrosin   and   various 
amino-acids,  fatty  acids,   ammonia,  phenol,  and  other 
bodies  containing   an   aromatic   nucleus,  gaseous  com- 
pounds of  sulphur,  etc.       In  the   main,  however,  the 
changes  of  organic  material  in  the  soil  fall  upon  the 
cellulose ;  it  loses  carbonic  acid,  marsh  gas,  hydrogen, 
etc.,  and   becomes  humus  with  a  gradually  increasing 
proportion  of  carbon ;  the  nitrogenous  materials  resist 
attack  more  than  the  carbohydrates,  and  hence  tend  to 
accumulate,  so  that  an  old  sample .  of  deep-seated  peat 
is  richer  in  nitrogen  than  a  more  recent  sample  taken 
from  nearer  the  surface.     Finally  the  humus  thus  pro- 
duced, which  may  be  called  peat,  is  essentially  an  acid 
product,  and  even  when   aerated   and    supplied    with 
mineral  materials  will  oxidise  with  extreme  slowness. 

The  Fixation  of  Free  Nitrogen. 

In  the  earliest  theories  regarding  the  nutrition  of  the 
plant  which  were  accepted  after  chemistry  had  become 
an  exact  science,  it  was  considered  that  the  plant 


vii.]  EARLY  THEORIES  175 

derived  its  nitrogen  from  the  humus  of  the  soil,  as, 
for  example,  in  de  Saussure's  statement  that  "  Plants 
receive  their  nitrogen  almost  entirely  by  the  absorption 
of  the  soluble  organic  substances."  This  view  was 
displaced  by  the  so-called  "  mineral  theory  "  of  Liebig, 
who,  in  laying  down  the  broad  principle  that  the  plant 
only  derived  certain  necessary  mineral  constituents,  its 
"ash,"  from  the  soil,  and  the  whole  of  its  carbon 
compounds  from  the  atmosphere,  was  led  to  regard 
the  nitrogen  as  well  as  the  other  combustible  matters 
of  the  plant  as  due  to  the  atmosphere,  largely  because 
of  the  exaggerated  estimate  which  then  prevailed  as  to 
the  amount  of  ammonia  from  the  air  that  was  brought 
down  in  the  rain.  Boussingault  had  already  shown,  by 
weighing  and  analysing  the  crops  on  his  own  farm  for 
six  separate  courses  of  rotation,  that  from  one-third  to 
one-half  more  nitrogen  was  removed  in  the  produce 
than  was  supplied  in  the  manure.  The  gain  of  nitrogen 
was  little  or  nothing  when  cereal  crops  only  were 
grown,  but  became  large  when  leguminous  crops  were 
introduced  into  the  rotation.  Liebig,  however,  con- 
sidered that  cereals,  as  well  as  the  other  plants,  were 
able  to  draw  their  ammonia  from  the  atmosphere, 
and  that,  provided  sufficient  mineral  plant  food  were 
forthcoming,  there  was  no  need  of  ammonia  compounds 
in  the  manure. 

This  view  of  Liebig's,  though  modified  later,  when 
he  admitted  that  cereals  must  obtain  their  nitrogen 
from  a  manurial  source  in  the  soil,  led  to  considerable 
investigation  of  the  source  of  the  nitrogen  in  the  plant. 
Boussingault  himself  carried  out  a  long  series  of 
laboratory  experiments,  in  which  weighed  seeds  con- 
taining a  known  proportion  of  nitrogen  were  grown 
in  artificial  soils  containing  no  nitrogen,  but  supplied 
with  the  ash  constituents  of  the  plant.  Care  was 


176      THE  LIVING  ORGANISMS  OF  THE  SOIL  [CHAP. 

taken  to  remove  all  ammonia  from  the  air  in  which 
the  plants  were  grown,  and  from  the  water  and 
carbonic  acid  supplied  to  them ;  finally,  after  growth 
had  ceased,  the  amount  of  nitrogen  in  the  plant  and 
in  the  soil  was  determined.  In  some  cases  a  known 
quantity  of  nitrogenous  compounds  was  supplied  as 
manure ;  but  all  the  results  went  to  show  that  there 
was  no  gain  of  combined  nitrogen  during  growth ;  the 
seed  and  manure  at  starting  contained  as  much  nitrogen 
as  was  found  in  the  plant  and  soil  at  the  end. 

Similar  experiments  were  carried  out  with  the 
utmost  precautions  by  the  Rothamsted  investigators, 
who  likewise  found  no  gain  of  nitrogen  by  the  plant 
from  the  atmosphere.  The  following  results,  obtained 
by  Lawes  and  Gilbert  in  1858,  will  serve  to  show 
the  agreement  between  the  nitrogen  supplied  and 
recovered : — 


a 

fl 

a 

a 

"a"0  » 

s  . 

sag 

"~  "O     • 

>H 

Si^  S 

Si  <6  "e 

0  w 

a*  5 

SD  ^  "c 

C  QQ 

O  gj)     g 

°~  JS 

iiS 

o'g  fl 

°s  * 

'3  3 

-*J           0 

Si8" 

o1-1 

•S  ^  S 

StgE 

O  . 

2           •"•" 

R 

» 

S5 

Wheat 

OO078 

00081 

+  00003 

00548 

00536 

-OOOI2 

Barley 

00057 

0-0058 

+  O-OOOI 

00496 

00464 

-00032 

Oats  . 

O-0063 

0-0056 

-00007 

00312 

OO2I6 

-00096 

Beans 

OO75O 

00757 

+  00007 

OO7II 

00655 

-00056 

Peas  . 

00188 

00167 

-OOO2I 

OO227 

O02II 

-00016 

Clover 

OO7I2 

00665 

-OO047 

Buckwheat 

OO2OO 

00182 

-OOOI8 

00308 

OO292 

-OOOI6 

It  has  sometimes  been  objected  that  the  plants  in 
these  experiments  made  such  a  poor  growth  as  compared 
with  their  normal  development  in  the  open  air  that  they 
never  attained  their  usual  power  of  fixing  nitrogen. 
However,  Hellriegel's  experiments  on  plants  which 
were  supplied  with  limited  amounts  of  nitrogen  showed 
that  growth  is  practically  proportional  to  the  supply  of 


VII.] 


FIXATION  OF  NITROGEN 


177 


nitrogen  as  long  as  that  is  below  the  maximum  required 
by  the  plant.  Field  experiments  at  Rothamsted  with 
leafy  crops  like  mangolds,  to  which  a  very  small  amount 
of  nitrogen  was  supplied  in  order  to  give  them  a  start, 
showed  that  the  increase  thus  produced  was  only  pro- 
portional to  the  nitrogen  supplied,  so  that  there  is  no 
evidence  that  even  a  plant  which  has  begun  to  grow 
vigorously  can  then  continue  its  development  by  taking 
nitrogen  from  the  atmosphere. 

From  all  these  experiments  the  conclusion  was 
drawn  that  cultivated  plants  are  unable  to  "fix" 
atmospheric  nitrogen,  but  obtain  this  indispensable 
element  in  a  combined  state  from  the  soil  together 
with  the  ash  constituents ;  and  such  was  the  opinion 
that  prevailed  for  something  like  thirty  years. 

Notwithstanding  the  conclusive  nature  of  all  the 
laboratory  experiments,  there  was  still  a  residuum  of 
facts  obtained  under  field  conditions  which  were  in- 
explicable on  the  theory  of  the  non-fixation  of  nitrogen, 
and  these  facts  were  chiefly  connected  with  the  growth 
of  leguminous  crops. 

Boussingault's  crop  statistics  have  already  been 
referred  to ;  the  following  table  gives  a  short  summary 
of  the  kind  of  results  he  obtained  : — 


NITROGEN. 

Kilos  per  hectare. 

Supplied  in 

Removed  in 

Manure. 

Crop. 

Wheat,  Wheat,  Fallow     

87-2 

82-8 

Potatoes,  Wheat,  Clover,  Wheat  or  Turnips, 

Oats     

202-2 

268-5 

Potatoes,  Wheat,  Clover,  Wheat 

182 

339 

Lucerne,  5  years      

1035 

The   amount   of    nitrogen    removed    was   equal   to 

M 


I7&      THE  LIVING  ORGANISMS  OF  THE  SOIL   [CHAP. 

that  supplied  only  when  wheat  was  grown,  but  became 
progressively  greater  the  more  frequently  leguminous 
crops  occupied  the  ground. 

At  Rothamsted  the  following  average  quantities  of 
nitrogen  were  removed  per  acre  per  annum  in  the 
crop,  when  mineral  manures  only  were  applied  : — 

Wheat  (24  years)  .  .  .  .        22-1 

Barley  (24  years)  ....         22-4 

Roots  (30  years)  .  .  .  .16-4 

Beans  (24  years,  only  21  years  in  Beans)  .         45-5 

Red  Clover  (22  years,  only  6  years  Clover)       .         39-8 

In  this  case  also  the  amount  of  nitrogen  in  the 
produce  was  much  increased  when  a  leguminous  crop 
was  grown. 

Another  of  the  Rothamsted  experiments  showed  still 
more  strikingly  the  accumulation  of  nitrogen  by  a 
leguminous  crop.  A  piece  of  land  which  had  been 
cropped  for  five  years  by  cereals,  without  any  nitro- 
genous manure,  was  divided  into  two  portions  in  1872, 
one  being  sown  with  barley  alone,  and  the  other  with 
clover  in  the  barley.  In  1873  barley  was  again  grown 
on  the  one  portion,  but  the  clover  on  the  other,  three 
cuttings  of  clover  being  obtained.  Finally,  in  1874, 
barley  was  grown  on  both  portions.  The  quantities  of 
nitrogen  removed  in  the  crops  of  1873  and  1874  are 
shown  in  the  table. 

NITROGEN  IN  CROP— LBS.  PER  ACRE. 


1873 
1873 

Barley        .       37-3 
Clover       .     151-3 

1874 
1874 

Barley         .       39-1 
Barley         .       69-4 

Thus,  the  barley  which  followed  clover  obtained 
30-3  Ibs.  more  nitrogen  than  the  barley  following  barley, 
though  the  previous  clover  crop  had  removed  1 14  Ibs. 
more  nitrogen  than  the  first  barley  crop.  An  analysis 


vii.]    LEGUMINOS&  AND  NITROGEN  FIXATION    179 

of  the  soil  was  made  in  1873,  after  the  clover  and  barley 
had  been  removed  ;  this  showed  down  to  the  depth  of 
9  inches  an  excess  of  nitrogen  in  the  clover  land,  despite 
the  larger  amount  which  had  been  removed  in  the  crop. 

In  Soil  after  Barley  .        0-1416  per  cent.  Nitrogen. 

In  Soil  after  Clover          .        0-1566       „  „ 

In  another  experiment,  land  which  had  previously 
grown  beans  and  then  been  fallow  for  five  years,  was 
sown  with  barley  and  clover  in  1883,  the  clover  being 
allowed  to  stand  in  1884  and  1885.  At  starting  the 
soil  was  analysed ;  the  surface  9  inches  contained  on 
an  average  2657  Ibs.  per  acre  of  nitrogen,  while  of 
nitrogen  as  nitric  acid  the  soil  only  contained  24-7  Ibs. 
per  acre  down  to  a  depth  of  6  feet.  As  a  result  of  the 
three  years  cropping  with  barley  and  clover,  and  then 
with  clover  only,  an  average  amount  of  319-5  Ibs.  of 
nitrogen  was  removed,  yet  the  soil  contained,  on 
analysis  at  the  end  of  the  experiment,  2832  Ibs.  of 
nitrogen  per  acre  in  the  top  9  inches,  or  a  gain  of 
175  Ibs.  per  acre  in  the  three  years,  making  a  total, 
with  the  crop  removed,  of  nearly  500  Ibs.  of  nitrogen 
per  acre  to  be  accounted  for. 

The  consideration  of  field  trials  of  this  description 
led  many  observers  to  think  that  there  still  might  be 
some  fixation  of  free  nitrogen,  particularly  by  legumi- 
nous plants.  Voelcker,  in  England,  when  discussing 
the  power  of  a  clover  crop  to  accumulate  nitrogen, 
expressed  the  opinion  that  the  atmosphere  furnishes 
nitrogenous  food  to  that  plant ;  in  France,  it  was 
maintained  by  Ville ;  Berthelot  also  brought  evidence 
to  show  that  the  soil  itself,  by  the  aid  of  its  micro- 
scopic vegetation,  assimilated  some  free  nitrogen.  Even 
in  the  laboratory  experiments,  some  of  Boussingault's 
results,  and  others  of  Atwater,  in  America,  showed  a 


i8o      THE  LIVING  ORGANISMS  OF  THE  SOIL    [CHAP. 

gain  of  nitrogen.     But  the  clearing  up   of  the  whole 
subject    came   with   the   publication,    in    1886,   of   the 
researches  of  Hellreigel  and  Wilfarth.     These   investi- 
gators   found   that   when   plants  were   grown    in   sand 
and    fed   with    nutrient  solutions,  the    Gramineae,  the 
Cruciferae,  the  Chenopodiaceae,  the  Polygonaceae,  grew 
almost    proportionally    to    the    amount    of    combined 
nitrogen  supplied ;    and,   if  this  were  absent,  nitrogen 
starvation  set  in  as  soon  as  the  nitrogen  of  the  seed 
was  exhausted.      With    the    Leguminosae,  however,   a 
plant    was  observed    sometimes    to    recover  from   the 
stage    of   nitrogen   starvation,   and   begin   a  luxurious 
growth   which   lasted   until   maturity,  though  no  com- 
bined nitrogen  was  supplied.     In  such  cases  the   root 
of  the  plant  was  always  found  to  be  set  with  the  little 
nodules  characteristic  of  the  roots  of  leguminous  plants 
when  growing  under  natural  conditions.    Further  experi- 
ments were  made  in  which  the  plants  were  grown  in 
sterile  sand,  but  as  soon  as  the  stage  of  nitrogen  hunger 
was   reached,   a  small  portion   of  a   watery  extract  of 
ordinary   cultivated    soil   was    added ;    whereupon,  the 
plants  receiving  the  extract  recovered  from  their  nitrogen 
starvation  and  grew  to  maturity,  assimilating  consider- 
able quantities  of  nitrogen.     The  renewed  growth  and 
the  assimilation  of  nitrogen  were  always  found  to  be 
attendant  upon  the  production  of  nodules  on  the  roots. 
The  nodules  were  found  to  be  full  of  bacteria,  to  which 
the   name  of  Pseudomonas  radicicola  has  been  given. 
They  could  only  be  produced  by  previous  infection  either 
by  an  extract  of  the  crushed  nodules  or  of  a  cultivated 
soil ;  in  some  cases  (lupins,  serradella)  only  by  soil  which 
had  previously  carried  the  same  crop. 

These  results,  though  not  at  first  accepted  by  Lawes 
and  Gilbert,  led  to  a  repetition  of  the  experiments, 
which  brought  out  the  fact  that  in  their  earlier 


VII.]  HOW  TS  NITROGEN  FIXATION  EFFECTED?    181 

trials  with  leguminous  plants  the  necessary  inocula- 
tion had  always  been  wanting  because  of  the  great 
care  that  had  been  taken  to  prevent  the  entry  of  any 
accidental  impurity.  Eventually,  both  at  Rothamsted 
and  by  other  investigators,  the  conclusions  of  Hellreigcl 
and  Wilfarth  were  confirmed,  that  when  leguminous 
plants  are  grown  under  sterile  conditions,  without  a 
supply  of  combined  nitrogen  there  is  very  limited 
growth,  no  formation  of  nodules,  and  no  gain  of  nitrogen. 
But  when  the  culture  is  seeded  with  soil  extract  there  is 
luxuriant  growth,  abundant  nodule  formation,  and  coin- 
cidently,  great  gain  of  nitrogen,  many  times  as  much  in 
the  products  of  growth  as  in  the  seed  sown.  Gilbert  also 
showed  that  there  is  a  gradual  accumulation  and  then 
withdrawal  of  nitrogen  from  the  nodules.  Lastly, 
Schloesing  fils  and  Laurent,  by  growing  Leguminosai  in 
closed  vessels,  and  analysing  the  air  before  and  after 
growth,  found  an  actual  disappearance  of  nitrogen  gas, 
agreeing  with  the  amount  gained  by  the  plant  during 
growth.  Thus,  a  conclusion  was  reached  that  the 
leguminous  plants  can  assimilate  and  fix  the  free 
nitrogen  of  the  atmosphere  by  the  aid  of  bacteria  living 
symbiotically  in  the  root  nodules, — a  conclusion  which 
served  to  explain,  not  only  the  discrepancies  in  the 
previous  experiments,  but  the  long-accumulated  experi- 
ence of  farmers  that  crops  like  clover  and  lucerne  enrich 
the  soil,  and  form  the  best  preparation  for  cereals  like 
wheat,  which  are  particularly  dependent  on  an  external 
supply  of  nitrogen.  The  mechanism  of  the  fixation  of 
free  nitrogen  is  still  incompletely  understood.  It  has 
been  found  possible  to  grow  these  bacteria  apart  from 
the  leguminous  plants,  if  they  are  cultivated  on  a  medium 
containing  only  a  trace  of  nitrogen  but  supplied  with 
the  ash  constituents  of  the  plant  and  also  with 
some  carbohydrate  like  dextrose  or  maltose.  The 


1 82      THE  LIVING  ORGANISMS  OF  THE  SOIL    [CHAP. 

quantities  of  nitrogen  fixed  in  this  way  are  always, 
however,  very  much  smaller  than  are  fixed  by  a 
leguminous  plant  on  whose  root  the  nodules  are  well 
developed.  To  fix  the  nitrogen,  some  expenditure  of 
energy  is  required,  and  this  is  derived  from  the 
combustion  of  carbohydrate  supplied  to  the  bacteria  by 
the  higher  plant ;  indeed  it  has  been  observed  that  the 
nitrogen  fixation  and  general  growth  of  the  Leguminosae 
is  stimulated  by  a  supply  of  sugar  or  other  carbohydrate 
to  the  soil.  The  organism,  Pseudomonas  radicicola, 
appears  to  be  capable  of  considerable  modifications ; 
in  the  nodules  it  forms  rather  large  rod  or  Y-shaped 
organisms,  but  if  an  active  subculture  be  obtained  by 
inoculation  from  a  nodule  into  a  non-nitrogenous 
medium  as  described  above,  excessively  minute  rod- 
shaped  organisms  appear,  generally  in  rapid  motion. 
It  is  in  this  form  they  are  supposed  to  exist  free  in 
the  soil,  and  it  has  been  shown  that  they  infect  the 
leguminous  host  by  getting  through  the  thin  walls  of 
the  root  hairs.  The  characteristic  Y  forms  have  also  been 
obtained  in  artificial  cultivations  by  introducing  certain 
substances  into  the  medium.  Much  investigation  has 
also  been  applied  to  the  question  of  whether  there 
is  only  one  kind  of  bacterium  living  in  symbiosis 
with  all  the  Leguminosae,  or  whether  there  is  not  a 
definite  race  appropriate  to  each  species  of  leguminous 
plant,  with  which  it  alone  can  bring  about  nitrogen 
fixation  to  the  full  extent.  The  earliest  investigations 
had  already  shown  that  lupins  and  serradella  did  not 
develop  nodules  when  infected  with  an  ordinary 
garden  soil,  but  only  when  an  extract  was  added 
from  a  sandy  soil  on  which  these  plants  had  been 
previously  grown  ;  and  Nobbe  brought  further  evidence 
to  show  that,  though  there  is  very  widely  distri- 
buted in  the  soil  an  organism  which  will  cause 


vii.]          CULTURES  FOR  SOIL  INOCULATION          183 

some  nodule  formation  and  fixation  of  nitrogen, 
yet  it  becomes  so  modified  by  growing  in  symbiosis 
with  the  different  leguminous  plants,  that  the  best 
results  are  only  obtained  when  each  species  is  directly 
infected  from  nodules  taken  from  the  same  kind  of  plant. 
Accordingly,  he  proceeded  to  the  introduction,  on  a 
commercial  scale,  of  pure  cultivations  on  a  gelatine 
medium  of  the  races  of  bacteria  appropriate  to  each  of 
the  leguminous  plants  grown  as  field  crops.  The  jelly, 
which  was  called  "  Nitragin,"  was  to  be  dissolved  in  a 
large  bulk  of  water  and  sprinkled  over  the  seed  before- 
sowing  ;  thus  ensuring  inoculation  with  the  appropriate 
organism,  which  might  not  happen  to  be  present  in  the 
soil.  Nitragin  failed  to  fulfil  the  expectations  which 
were  formed  at  its  introduction,  partly  because  of  the 
nitrogenous  character  of  the  medium,  in  consequence  of 
which  the  organisms  possessed  very  little  vitality  or 
power  of  fixing  nitrogen.  Since  that  time,  however, 
several  other  methods  of  cultivating  the  organism  for 
inoculation  purposes  have  been  introduced,  either  by 
growing  it  on  an  agar  jelly,  which  contains  practically 
no  nitrogen  (Hiltner),  by  drying  up  cotton  wool  which 
has  been  soaked  in  an  active  liquid  culture  (Moore), 
or  by  drying  soil  which  has  been  treated  in  the  same 
way.  The  culture  thus  obtained  is  added  to  a  large 
bulk  of  water,  containing  a  little  separated  milk  to 
protect  the  organisms  from  substances  excreted  during 
germination,  and  the  seed  is  dipped  into  it  and 
allowed  to  dry  before  sowing.  The  culture  may 
also  be  sprayed  over  the  ground  or  absorbed  by  a 
large  quantity  of  earth  which  is  aftenvards  sown.  The 
results  of  such  inoculation  are  very  conflicting  ;  where  the 
land  has  been  regularly  under  cultivation  and  has  carried 
the  leguminous  crop  in  question  many  times  previously, 
nodules  are  practically  always  formed  whether  the  seed 


1 84      THE  LIVING  ORGANISMS  OF  THE  SOIL    [CHAP 

be  inoculated  or  not.  In  such  cases  inoculation  can 
only  be  beneficial  if  the  bacteria  introduced  either 
belong  to  a  more  vigorous  race  of  nitrogen  fixers 
than  those  normally  present  in  the  soil  or  are  more 
specifically  adapted  to  that  particular  crop.  It  has  not 
as  yet  been  conclusively  demonstrated  that  such  im- 
proved races  can  be  cultivated  in  the  laboratory,  or  that 
they  can  maintain  themselves  in  the  soil  in  competition 
with  the  kindred  organisms  already  present  It  should, 
moreover,  be  borne  in  mind  that  even  if  such  improved 
races  of  the  nodule-forming  organism  can  be  introduced 
to  the  plant,  the  improvement  they  can  produce  in  the 
yield  is  likely  to  be  something  of  the  order  of  a  ten  per 
cent,  increase,  a  gain  which  is  only  really  perceptible 
after  careful  and  continued  field  experiments,  and  one  not 
to  be  detected  by  the  ordinary  farmer's  eye.  Of  a  very 
different  order  are  the  results  attained  by  inoculation 
when  the  land  contains  none  of  the  appropriate  organ- 
isms ;  inoculation  will  then  change  a  stunted,  sickly 
looking  growth  into  a  profitable  crop.  It  is  only  in 
special  cases  that  land  devoid  of  the  nodule  organisms  is 
to  be  met  with,  most  commonly  when  land  is  being 
brought  under  cultivation  for  the  first  time,  as  in  break- 
ing up  a  virgin  soil  or  in  reclaiming  heath  and  bog  land. 
Such  peaty  and  heathy  soils,  which  are  devoid  of 
carbonate  of  lime,  rarely  carry  any  leguminous  plants,  the 
bacteria  in  the  nodules  of  which  are  of  much  service 
to  farm  crops  like  clover  and  lucerne ;  when  such  land 
has  been  reclaimed  and  limed  an  inoculation  is  advisable 
before  sowing  a  leguminous  crop. 

Similarly  when  the  cultivation  of  such  leguminous 
crops  as  lucerne  or  even  sainfoin  is  being  extended  into 
districts  where  they  have  not  been  grown  previously,  an 
inoculation  is  often  necessary  before  the  roots  will  nodu- 
late  freely  and  the  plant  make  its  proper  growth.  Lucerne 


vii.]  GREEN  MANURING  185 

grown  for  the  first  time  on  heavy  land  in  a  new  district 
has  been  observed  to  fail  completely,  the  failure  being 
attended  by  a  complete  absence  of  nodules  from  the  roots. 
Inoculation  with  soil  from  a  field  which  has  pre- 
viously grown  the  crop  about  to  be  sown  has  often 
proved  a  signal  success  in  reclaiming  the  poor  heath 
lands  of  East  Prussia,  by  the  system  of  green  manuring 
worked  out  by  Dr  Schultz  at  Lupitz.  Very  large 
areas  of  barren  sandy  heath  land  have  been  re- 
claimed and  rendered  fit  for  the  cultivation  of  the 
ordinary  crop  by  a  system  of  growing  lupins  and 
ploughing  in  the  green  crop.  Mineral  manures  alone 
are  employed,  latterly  basic  slag  and  the  Stassfurt 
potash  salts ;  the  lupins  accumulate  nitrogen  from 
the  atmosphere,  thus  gradually  there  is  built  up  both 
humus  to  bind  together  the  loose  sand  and  make  it 
retentive  of  moisture,  and  also  a  store  of  nitrogen  for 
the  nutrition  of  succeeding  crops.  The  soil  of  a  field 
growing  lupins  every  year  from  1865  was  found  in 
1880  to  contain  0-087  per  cent,  of  nitrogen  in  the 
surface  8  inches,  as  compared  with  0-027  per  cent, 
in  an  adjoining  pasture.  By  1891  the  proportion  of 
nitrogen  had  increased  to  0-177  Per  cent.,  despite  the 
annual  removal  of  the  lupin  crop  and  the  fact  that 
the  manuring  had  been  with  phosphates  and  potash 
only.  It  is  in  reclaiming  these  heath  lands  which 
have  not  previously  been  under  cultivation,  nor,  in 
many  cases,  carried  any  leguminous  vegetation  what- 
ever, that  soil  inoculation  from  land  previously  cultivated 
has  given  successful  results.  Dr  Salfeld  of  Hanover  has 
recorded  several  cases  of  the  successful  cultivation  on  a 
large  scale  of  various  leguminous  plants,  beans,  clover, 
serradella,  lupins,  only  after  previous  inoculation  with 
soil.  The  experiments  were  made  on  both  peaty  (moor) 
and  sandy  soils,  on  which,  without  inoculation,  legumin- 


1 86       THE  LIVING  ORGANISMS  OF  THE  SOIL  [CHAP. 

ous  plants  made  but  little  growth  and  developed  no 
nodules.  Success  followed  when  about  8  cwt.  per  acre  of 
soil  from  a  field  which  had  previously  carried  the  crop  in 
question  were  sown  broadcast  over  the  land  in  April, 
and  harrowed  in  just  before  seeding.  In  one  case,  over 
7  tons  per  acre  of  green  serradella  were  grown  where 
the  land  had  been  treated  with  8  cwt.  of  soil  from  an 
old  serradella  field,  whereas  the  crop  failed  after  germi- 
nation where  no  inoculation  had  been  practised. 

Fixation  of  Free  Nitrogen  by  the  Soil. 

As  already  indicated,  Berthelot  attributed  to  the 
soil  itself  the  power  of  fixing  a  small  quantity  of  atmo- 
spheric nitrogen,  a  power  which  was  lost  when  the  soil 
was  sterilised  and  maintained  under  conditions  prevent- 
ing infection.  This  gain  of  nitrogen  was  independent 
of  the  small  amount  of  ammonia  absorbed  by  soil  from 
ordinary  air,  which  always  contains  a  trace  of  ammonia  ; 
and  at  first  it  was  attributed  to  the  microscopic  green 
algae  which  clothe  the  surface  of  ordinary  moist  soil. 
The  experiments  of  Kossowitsch,  and  of  Kriiger  and 
Schneidewind,  have,  however,  shown  that  the  growth  of 
pure  cultures  of  these  algae  is  dependent  on  a  supply 
of  combined  nitrogen,  and  that  no  fixation  of  free 
nitrogen  takes  place  whether  the  algae  growth  be  small 
or  large.  It  is  possible,  however,  that  they  may  live 
in  symbiosis  with  nitrogen-fixing  bacteria  and  supply 
the  carbohydrate,  by  the  combustion  of  which  the 
energy  needed  for  the  fixation  of  nitrogen  by  the  bacteria 
is  obtained.  More  recently,  however,  several  organisms 
have  been  isolated  from  the  soil  which  are  capable  when 
growing  in  a  free  state  of  fixing  nitrogen  drawn  from  the 
atmosphere,  and  it  is  to  these  that  the  gains  of  nitrogen 
observed  by  Berthelot  must  be  attributed.  Winogradsky 
was  the  first  to  isolate  an  organism  of  this  type,  which, 


vii.]  FIXATION  WITHOUT  LEGUMINOUS  PLANTS  187 

when  grown  under  anaerobic  conditions  and  supplied 
with  soluble  carbohydrate,  breaks  the  latter  down  with 
the  formation  of  butyric  and  other  acids,  and  at  the  same 
time  draws  some  of  the  gaseous  nitrogen  present  into  com- 
bination. This  particular  organism  Clostridium  Pastori- 
anum  is  very  widely  diffused  and  can  readily  be  isolated 
from  pond  mud  and  similar  material,  where  organic  matter 
is  decaying  under  comparatively  anaerobic  conditions. 
The  extent  of  the  nitrogen  fixation  is,  however,  small ; 
in  the  laboratory  not  more  than  2  to  3  mg.  of  nitrogen 
are  brought  into  combination  for  each  gram  of  carbo- 
hydrate oxidised.  By  far  the  most  effective  of  the 
nitrogen-fixing  bacteria  that  are  free  in  the  soil  is  a  large 
organism,  named  by  its  discoverer,  Beijerinck,  Azotobacier 
chroococcum.  It  may  be  easily  isolated  from  most  soils 
by  adding  a  small  portion  of  soil  to  50  c.c.  of  a  culture 
medium  containing  per  litre  10  grams  of  mannite  or 
glucose,  0-2  gram  each  of  potassium  phosphate,  mag- 
nesium sulphate,  and  sodium  chloride,  and  o  i  gram  of 
calcium  sulphate,  half  a  gram  of  calcium  carbonate  being 
also  added  to  each  flask.  The  solution  and  its  flask 
and  plug  of  cotton  wool  are  previously  sterilised  by  heat. 
After  inoculation,  the  flask  is  placed  in  an  incubator, 
and  after  a  week's  time  a  considerable  fermentation  will 
be  observed  to  have  taken  place,  attended  by  the 
evolution  of  gas  and  the  formation  of  a  brown  scum  upon 
the  surface.  By  making  a  subculture  in  a  similar 
medium,  inoculated  with  a  trace  of  the  brown  scum,  a 
fairly  pure  growth  of  the  Azotobacter  can  be  obtained 
for  examination,  or  the  amount  of  nitrogen  fixed  may 
be  determined  by  Kjeldahling  the  contents  of  the 
flask. 

Azotobacter  chroococcuni  is  a  large  oval  organism,  4  to 
5  /JL  in  length  and  3  /j.  in  width,  which  differs  from  most 
bacteria  in  containing  glycogen,  so  that  it  stains  a  deep 


1 88       THE  LIVING  ORGANISMS  OF  THE  SOIL  [CHAP. 

brown  colour  with  a  solution  of  iodine,  a  method  which 
is  convenient  for  the  observation  of  the  organism.  It  is 
aerobic,  and  is,  in  fact,  a  strong  oxidising  agent,  the 
dextrose  or  other  carbohydrate  which  it  requires  being 
converted  by  it  into  carbon  dioxide  and  water,  together 
with  small  quantities  of  lactic  and  acetic  acids,  alcohol, 
and  sometimes  butyric  acid. 

A  very  characteristic  bye-product  is  the  dark  brown 
or  black  pigment  from  which  the  organism  derives  its 
specific  name,  a  pigment  which  may  play  its  part  in  the 
usual  coloration  of  humus. 

As  a  rule,  about  9  or  10  mg.  of  nitrogen  are  fixed 
for  each  gram  of  carbohydrate  oxidised,  but  the  ratio 
obtained  varies  considerably  under  different  conditions ; 
cultures  which  have  been  repeatedly  transferred,  being, 
as  a  rule,  less  effective  than  the  impure  culture  derived 
directly  from  the  soil. 

Azotobacter  chroococcum  and  its  kindred  forms  are 
widely  distributed  in  soils  from  all  parts  of  the  world ; 
it  has  been  found  in  most  cultivated  soils,  and  the  author 
has  observed  it  in  virgin  soils  from  East  Africa,  India, 
New  Zealand,  Egypt,  Russia,  Monte  Video,  Ohio,  and 
Sarawak. 

It  is,  however,  not  to  be  discovered  in  acid  soils  ;  the 
presence  of  calcium  carbonate  appears  to  be  essential  to 
its  development.  Certain  minor  differences  are  to  be 
seen  in  the  Azotobacter  organisms  present  in  the  soil 
from  different  parts  of  the  world.  From  tropical  and 
semi-tropical  soils  in  East  Africa,  for  example,  a  form 
has  been  isolated  which  is  a  very  effective  fixer  of 
nitrogen,  but  which  differs  from  the  normal  in  not  giving 
rise  to  the  brown  pigment ;  another  form,  again,  from 
Monte  Video  gives  rise  to  a  green  fluorescence  in  the 
culture  medium. 

The  amount  of  nitrogen  fixed  by  Azotobacter  may 


vii.]      NITROGEN  FIXATION  IN  VIRGIN  SOILS       189 

easily  be  rendered  evident  by  an  increased  yield  of 
crop.  Koch  treated  soil  in  pots  with  large  quantities  of 
sugar,  2  per  cent,  4  per  cent,  and  even  more  of  dextrose, 
and  then  sowed  oats,  buckwheat,  etc.  At  first  the  sugar 
was  injurious,  and  the  first  crop  suffered  in  consequence  ; 
but  the  proportion  of  nitrogen  in  the  soil  increased,  and 
the  second  and  third  crops  were  far  greater  than  those 
in  the  check  plots  of  untreated  soil.  When  the  soil, 
after  the  application  of  the  sugar,  was  placed  in  an 
incubator  for  a  month,  in  order  to  complete  the  oxidation 
of  the  sugar,  the  increased  yield  due  to  nitrogen  fixation 
was  also  seen  in  the  first  crop. 

To  the  Azotobacter  and  kindred  organisms  must 
certainly  be  ascribed  a  large  part  in  preparing  and 
maintaining  the  world's  stock  of  combined  nitrogen.  It 
is  customary  to  regard  such  virgin  soils  as  the  black  soils 
of  the  Russian  Steppes,  of  Manitoba,  and  of  the  Argen- 
tina, as  rich  in  nitrogen  because  of  the  accumulation  of  the 
vegetable  debris  of  many  epochs ;  but  since  plants  other 
than  the  Leguminosae  do  not  fix  nitrogen  themselves, 
there  could  in  this  way  be  no  addition  to  the  original 
stock,  which  would  only  circulate  from  the  soil  to  the 
plant  and  back  to  the  soil  again.  Under  such  conditions, 
however,  there  is  a  continual  addition  to  the  soil  of  the 
carbon  compounds  which  the  plant  derives  from  the 
atmosphere,  and  this  is  material  which  the  Azotobacter 
can  oxidise,  and  so  derive  the  energy  required  for  the 
fixation  of  nitrogen.  It  is  the  constant  return  to  the  soil 
of  oxidisable  organic  matter  which  differentiates  the 
wild  from  the  cultivated  land,  and  renders  possible  the 
long-continued  storing  up  of  nitrogen  in  the  virgin  soils. 
Interesting  evidence  on  this  point  may  be  derived 
from  the  Rothamsted  experiments ;  on  the  Broadbalk 
wheatfield  the  unmanured  plot  has,  during  the  fifty 
years  1844-93  yielded  a  crop  containing  on  the  average 


i go       THE  LIVING  ORGANISMS  OF  THE  SOIL  [CHAP. 

17  Ibs.  of  nitrogen  per  acre  per  annum.  Analyses  of 
the  soil  at  the  beginning  and  end  of  the  period  showed 
a  decline  in  the  amount  of  nitrogen  equivalent  to  a 
removal  of  12  Ibs.  per  acre  per  annum,  and  the  rainfall 
is  known  to  bring  down  between  4  and  5  Ibs.  per  acre 
per  annum.  The  annual  withdrawal  in  the  crop  would 
thus  be  closely  balanced  by  the  loss  experienced  by  the 
soil  and  the  additions,  were  there  not  other  unknown 
withdrawals  in  the  weeds  which  are  removed  from  the 
plot,  and  in  the  nitrates  which  are  washed  down  into  the 
subsoil  and  the  drains.  Doubtless,  neither  of  these  two 
withdrawals  are  large,  but  because  of  their  existence,  un- 
balanced by  any  corresponding  falling  off  in  the  nitrogen 
content  of  the  soil,  it  must  be  concluded  that  even  on 
the  arable  land  some  small  restorative  action  is  going 
on.  A  portion,  however,  of  the  same  field  has  been 
covered  with  a  wild  vegetation  of  weeds  and  grasses  for 
the  last  twenty-five  years,  and  this  is  never  cut  or 
harvested,  so  that  all  the  debris  fall  back  on  the  land 
just  as  it  would  on  a  virgin  soil.  Analysis  of  samples  of 
this  soil  taken  in  1881,  when  it  ceased  to  be  under  cultiva- 
tion, and  in  1904,  showed  an  annual  accumulation  of 
nitrogen  of  more  than  100  Ibs.  per  acre.  The  enormous 
difference  in  the  fixation  on  this  plot  as  compared  with 
the  unmanured  plot  carrying  wheat,  must  be  set  down 
to  the  difference  in  the  supply  of  non-nitrogenous  carbon 
compounds  to  the  two  plots ;  in  the  one  case  the  wheat 
is  all  removed  except  a  small  portion  of  root  and  stubble  ; 
in  the  other  the  whole  of  the  vegetable  growth  falls  back 
on  the  land.  The  wild  vegetation  on  this  plot  did  include 
a  considerable  proportion  of  leguminous  plants,  but  a 
similar,  though  smaller  accumulation  of  nitrogen  was 
observed  in  another  plot  of  land  which  had  been  allowed 
to  run  wild  in  the  same  manner,  but  which  carried  no 
leguminous  vegetation  in  consequence  of  the  small 


vii.]  FORMATION  OF  NITRATES  IN  SOIL  191 

amount  of  calcium  carbonate  in  the  soil.  These  two 
plots  present  a  very  close  parallel  to  the  actions  which  must 
have  been  taking  place  in  all  virgin  soils  where  the  soil 
similarly  contains  the  Azotobacter  organism.  Doubtless 
also  some  of  the  value  of  laying  down  land  to  temporary 
pasture  must  be  due  to  the  accumulation  of  nitrogen  by 
the  same  agency,  because  we  know  that  land  under  grass 
accumulates  carbon  compounds  from  the  roots  and 
stubble  that  is  not  removed  during  grazing. 

Nitrification. 

It  has  long  been  known  that  when  any  organic  com- 
pound of  nitrogen  is  applied  to  the  soil  it  becomes  event- 
ually oxidised  to  a  nitrate,  which  is  practically  the  only 
compound  of  nitrogen  taken  up  by  cultivated  plants,  the 
Leguminosae  excepted.  The  potassium  nitrate  collected 
from  Indian  soils,  the  calcium  nitrate  made  artifically  in 
nitre  beds  in  Europe,  owe  their  origin  to  this  oxidation 
of  organic  compounds  of  nitrogen.  That  the  process 
was  a  biological  one  was  first  indicated  by  M tiller  in 
1873,  DUt  any  widespead  recognition  of  the  fact  did  not 
take  place  before  the  work  of  Schloesing  and  Muntz  in 
1877.  These  investigators  showed  that  the  formation 
of  nitrates  in  the  soil  ceased  at  temperatures  below  5° 
and  above  55°  C,  that  it  could  be  stopped  by  chloroform 
vapour  and  similar  antiseptics,  and  that  the  soil  lost 
entirely  its  power  of  nitrification  if  it  were  heated  to  the 
temperature  of  boiling  water.  The  investigations  of 
VVarington  confirmed  these  results,  and  brought  to  light 
the  further  fact  that  there  were  two  stages  in  the  oxida- 
tion process,  one  being  the  formation  of  a  nitrite, 
followed  by  the  conversion  of  this  nitrite  into  the  com- 
pletely oxidised  product.  It  was  found  possible  to  obtain 
cultures  which  would  only  push  the  oxidation  to  the 
nitrite  stage,  thus  indicating  that  there  must  be  at  least 


192      THE  LIVING  ORGANISMS  OF  THE  SOIL    [CHAP. 

two  organisms  concerned  in  the  complete  nitrification 
process.  The  further  study  of  the  organisms  was  for 
a  long  time  hindered  by  the  fact  that  they  could  not  be 
got  to  grow  upon  the  gelatinous  media  employed  in  the 
ordinary  methods  of  isolating  specific  bacteria ;  and 
though  P.  F.  Frankland,  by  a  dilution  method,  succeeded 
in  isolating  and  describing  a  nitrifying  bacterium,  it  was 
not  until  1890  that  Winogradsky  cleared  up  the  problem. 
He  prepared  a  solid  nutritive  medium  containing  no 
organic  matter  but  with  silica  in  its  gelatinous  form  as  a 
basis,  and  thus  was  able  to  separate  nitrifying  bacteria 
from  the  large  number  of  other  species  simultaneously 
present  in  the  soil.  Winogradsky  was  able  to  isolate 
two  species  of  bacteria  capable  of  transforming  ammonia 
compounds  into  nitrites.  One  of  these,  termed  Nitro- 
somonas  europcea,  was  obtained  from  all  the  soils  of 
the  old  world  he  examined ;  the  other,  ascribed  to 
the  genus  Nitrococcus,  was  peculiar  to  the  soils  of 
America  and  Australia.  The  former  occurs  both  as  a 
single,  free-swimming  form,  and  clustered  together  in  a 
colony  or  zooglcea  state. 

Finally,  there  appears  to  be  one  type  of  organism 
only,  included  in  the  genus  Nitrobacter,  which,  oxi- 
dises the  nitrites  to  nitrates.  Winogradsky  and  other 
observers  have  worked  out  the  conditions  of  life  of 
these  nitrifying  organisms — the  limits  of  temperature 
for  their  growth,  5°  and  55°  C,  have  already  been  given, 
the  optimum  temperature  is  about  37°  C.  Their  action 
is  much  restrained  by  the  presence  of  organic  matter, 
or  any  quantity  of  alkaline  carbonates  or  chlorides ;  at 
the  same  time,  some  base  *  must  be  present  to  combine 

*  Instruction  sur  la  fabrication  du  nitre : — Par  les  rt'gisseurs 
gt'rdraux  des  poudres  et  salt  pftres,  1777,  "  Elles  doivent  1'etre 
toujours  avec  une  addition  de  terre  calcaire  qui  puisse  servir  de 
base  a  1'acide  nitreux." 


vil.]     CONDITIONS  FAVOURING  NITRIFICATION    193 

with  the  nitrous  or  nitric  acids  produced,  for  nitrification 
ceases  as  soon  as  the  medium  becomes  at  all  acid. 
While  calcium  carbonate  is  the  substance  which, 
as  a  rule,  is  effective  to  this  end,  many  organic  salts  will 
also  supply  the  necessary  base.  Ammonium  salts  of 
the  strong  acids  will  not  nitrify  directly  in  the  absence 
of  a  base,  and  the  function  of  the  calcium  or  magnesium 
carbonate  is  usually  added  to  form,  by  double  decomposi- 
tion, ammonium  carbonate,  which  the  nitrifying  organ- 
isms can  attack.  The  complex  salts  formed  by  the 
interaction  of  the  zeolites  of  clay  with  ammonium 
salts  can  be  nitrified  directly,  but  not,  however,  the 
ammonium  humate  formed  by  the  corresponding 
interaction  of  ammonium  salts  and  humus.  Humus 
itself  does  not  inhibit  nitrification,  and,  indeed,  the 
organisms  can  be  brought  to  tolerate  considerable 
quantities  of  other  organic  matter,  by  transferring 
them  into  successively  stronger  solutions.  The  organ- 
isms are  able  to  obtain  the  carbon  necessary  to  their 
growth  from  carbonates  in  the  culture  medium  or 
carbonic  acid  in  the  air ;  the  energy  necessary  to 
decompose  the  carbon  dioxide  and  fix  the  carbon  is 
derived  from  the  oxidation  of  the  ammonia,  about 
35  parts  of  nitrogen  being  oxidised  for  each  part  of 
carbon  that  is  fixed.  The  nitrifying  organisms  are 
chiefly  confined  to  the  cultivated  surface  layer  of 
the  soil.  Warington  found  that,  in  the  close-textured 
Rothamsted  soil  they  were  by  no  means  uniformly 
distributed  below  the  top  9  inches,  and  that  they  were 
never  present,  except  accidentally,  in  the  subsoil  below 
a  depth  of  2  feet.  It  has  also  been  shown  that  they 
are  entirely  absent  from  many  heath  and  moor  soils, 
even  in  the  surface  layer.  They  are  abundantly  found 
in  the  water  of  shallow  wells  and  rivers. 

Summing  up  the  above  facts,  it  is  seen  that  for  the 

N 


194      THE  LIVING  ORGANISMS  OF  THE  SOIL    [CHAP. 

active  production  of  nitrates  from  the  organic  com- 
pounds of  nitrogen  present  in  the  soil — and  this  is 
necessary  if  the  crop  is  to  be  kept  supplied  with  the 
nitrogen  required  for  its  growth — the  following  condi- 
tions are  requisite  : — The  presence  of  the  nitrifying 
organisms  in  sufficient  quantities,  a  certain  degree 
of  temperature,  darkness,  sufficient  moisture  for  the 
development  of  the  bacteria,  free  aeration  of  the  soil 
to  supply  the  oxygen  necessary,  and  a  base  to  neutralise 
the  acids  as  they  are  produced. 

The  scanty  number  of  nitrifying  bacteria  in  any 
subsoil  below  the  cultivated  layer  helps  to  explain 
both  its  sterile  nature  when  brought  to  the  surface,  and 
the  difficulty  and  length  of  time  required  to  develop 
a  state  of  fertility,  especially  when  dealing  with  a 
clay  soil  in  which  percolation  and  aeration  have  been 
deficient. 

The  effect  of  a  low  temperature  in  checking  the 
formation  of  nitrates  is  well  seen  in  the  way  the  growing 
corn  turns  yellow  through  nitrogen  starvation  whenever 
a  cold  and  drying  north-east  wind  chills  the  ground 
in  spring :  the  bright  green  colour  returns  as  soon  as 
warmer  and  moister  soil  conditions  restore  the  activity 
of  the  nitrifying  bacteria  in  the  surface  layer.  King 
found  in  the  top  foot  of  soil  when  oats  were  turning 
yellow  only  0-026  parts  of  nitric  nitrogen  per  million 
of  dry  soil,  whereas  in  soil  where  the  oats  were  green 
on  the  same  date  there  was  0-255  parts  of  nitric  nitrogen 
per  million.  The  greater  warmth  of  a  light  soil  also 
causes  it  to  form  nitrates  quickly  in  the  spring,  and  so 
assists  in  producing  an  early  growth. 

But  in  obtaining  early  crops,  even  when  the  land  is 
rich,  a  dressing  of  ready-formed  nitrate  is  often  of  the 
greatest  assistance,  for  the  development  of  very  early 
crops  may  easily  outstrip  the  rate  at  which  the  nitrates 


[To  tact   pni/c  I!).1!. 


vii.]  NITRATES  IN  SOIL  195 

they  require  can  be  formed  in  the  still  unwarmed  soil. 
Nitrates  are  much  more  freely  formed  in  the  summer 
than  in  the  winter,  and  as  they  are  not  retained  by 
the  soil,  they  may  easily  be  washed  away  when  the 
crop  has  been  removed,  unless  weeds  or  a  catch  crop 
sown  to  that  end  are  present  to  take  up  the  nitrates 
and  store  them  as  organic  compounds  of  nitrogen  for 
the  future  enrichment  of  the  land. 

The  need  for  aeration  in  connection  with  the  nitrify- 
ing process  has  already  been  alluded  to  when  discussing 
drainage :  all  processes  of  working  and  cultivating  the 
soil  assist  nitrification,  both  by  the  thorough  aeration 
they  effect,  and  by  the  mere  mechanical  distribution 
of  the  bacteria  into  new  quarters,  where  there  are  fresh 
food  supplies.  In  some  experiments  of  Deherain's  he 
found  that  the  drainage  water  from  pots  of  cultivated 
soil,  which  had  been  sent  from  a  distance,  and  thus 
much  knocked  about  in  travelling  and  filling  into  the 
pot,  contained  as  much  as  466  to  664  parts  of  nitrogen 
as  nitric  acid  per  million.  The  drainage  water  from  the 
Rothamsted  wheat  plots  contains  only  from  10  to  20 
parts  per  million ;  even  the  cement  tanks  at  Grignon, 
2  metres  cube,  into  which  the  soil  had  been  filled,  gave 
drainage  water  containing  only  39  parts  of  nitric 
nitrogen  per  million.  In  another  experiment  a  quantity 
of  soil  was  thrown  upon  a  floor,  and  worked  about 
daily  for  six  weeks ;  on  analysis  it  contained  0-05 1 
per  cent,  of  nitric  nitrogen,  as  against  -002  per  cent,  of 
nitric  nitrogen  in  the  same  soil  left  in  situ.  The  diagram 
(Fig.  14),  due  to  King,  shows  the  dependence  of  nitrate 
production  on  temperature  and  the  cultivation  of  the 
soil.  The  lower  curve  shows  the  amount  of  nitrate 
in  parts  per  million  in  dry  soil  in  the  top  foot  of  land, 
which  was  not  being  cultivated  because  it  carried  clover 
and  oats.  The  upper  curve  shows  the  same  results 


196       THE  LIVING  ORGANISMS  OF  THE  SOIL  [CHAP. 

obtained  on  well-tilled  land  carrying  maize  and  potatoes. 
On  the  cultivated  land  the  proportion  of  nitrates  rises 
rapidly  until  the  end  of  June,  when  the  crop  begins  to 
draw  freely  upon  them,  and  reduces  them  to  a  minimum 
throughout  August  and  September. 

One  of  the  best  examples  of  the  manner  in  which 
the  thorough  working  and  aeration  of  a  warm  soil 
promotes  nitrification  is  seen  in  the  management  of 
the  turnip  crop  as  usually  grown  in  this  country. 
Though  shallow-rooted,  and  taking  away  large  quan- 
tities of  nitrogen  per  acre,  it  is  usually  grown  with 
but  little  nitrogenous  manure ;  phosphates  with  a  little 
dung  or  with  a  comparatively  small  nitrogenous  dress- 
ing, being  sufficient.  The  rest  of  the  nitrogen  is  derived 
from  the  rapid  production  of  nitrates,  due  to  the  very 
thorough  working  of  the  soil  in  the  warm  season  of  the 
year  that  is  characteristic  of  the  cultivation  of  the  turnip 
crop.  The  production  of  nitrates  by  cultivation  for 
the  benefit  of  a  succeeding  crop  by  bare  fallowing, 
or  of  an  adjoining  crop  as  in  the  Lois-Weedon  system 
of  alternate  husbandry,  has  been  already  alluded  to. 
At  Rothamsted,  nearly  60  Ibs.  per  acre  of  nitric  nitrogen 
were  found  in  October  in  the  top  27  inches  of  soil 
that  had  been  fallowed,  as  against  about  half  that 
amount  in  land  which  had  been  under  crop.  The  un- 
manured  alternate  wheat  and  fallow  plots  showed  in 
September  1878  to  a  depth  of  18  inches  33-7  Ibs.  of 
nitric  nitrogen  per  acre  after  fallow,  and  only  2-6  Ibs. 
after  wheat.  In  land  occupied  by  cereal  crops  the 
drainage  waters  show  that  there  is  practically  no  nitrate 
left  in  the  soil  by  May,  or,  at  the  latest,  June ;  they 
reappear  again  towards  the  end  of  July  or  in  August, 
and  after  harvest,  if  rain  falls,  and  especially  if  the  land 
be  ploughed,  nitrification  becomes  very  active.  It 
depends  upon  the  rainfall  of  the  autumn  and  winter 


VIL]  LOSSES  OF  SOIL  NITROGEN  197 

whether  these  nitrates,  formed  after  harvest,  are  retained 
for  the  succeeding  crop  or  are  washed  out  of  the  soil. 
To  sum  up,  increased  nitrification,  together  with  the 
conservation  of  soil  moisture  and  the  warming  of  the 
surface  soil,  are  among  the  chief  benefits  derived  from 
all  forms  of  surface  cultivation. 

As  so  much  of  the  fertility  of  a  soil  must  depend  on 
the  number  of  nitrifying  organisms  it  contains,  attempts 
have  been  made  to  compare  soils  in  this  respect,  by 
seeding  small  quantities  of  them  into  a  standard  solu- 
tion capable  of  nitrification  and  determining  the  amount 
of  nitric  acid  formed  after  a  given  time.  Although  con- 
siderable differences  are  seen  in  the  action  of  different 
soils,  satisfactory  quantitative  results  have  not  yet 
been  obtained,  because  of  difficulties  in  the  way  of 
drawing  strictly  comparable  samples  of  the  soils, 
and  the  uncertainty  still  attaching  to  the  amount 
which  should  be  used  for  inoculation  or  the  best  period 
of  incubation. 


Denitrification. 

The  term  denitrification  is  most  properly  applied 
to  the  reduction  of  nitrates  to  nitrites,  ammonia,  or 
particularly  to  gaseous  nitrogen,  which  is  brought 
about  by  bacterial  action  under  certain  conditions. 
Of  late,  however,  the  term  has  been  more  loosely 
used  to  denote  any  bacterial  change  which  results 
in  the  formation  of  gaseous  nitrogen,  whether  de- 
rived from  nitrates,  ammonia,  or  organic  compounds 
of  nitrogen. 

Angus  Smith  was  the  first  to  observe  the  evolution 
of  gas  from  a  decomposing  organic  solution  containing 
nitrates,  which  were  destroyed  in  the  process.  Other 


198       THE  LIVING  ORGANISMS  OF  THE  SOIL   [CHAP. 

observers,  particularly  Dehdrain  and  Maquenne  (1882), 
with  regard  to  soils,  confirmed  these  results  and  showed 
that  they  were  due  to  bacterial  action.  In  a  paper 
published  in  1882,  Warington  described  an  experiment 
in  which  sodium  nitrate  was  applied  to  a  soil  saturated 
with  water ;  after  standing  for  a  week,  the  nitrate  was 
washed  out  of  the  soil,  part  of  it  had  disappeared,  and 
part  had  become  nitrite.  The  total  of  both  nitric  and 
nitrous  nitrogen  only  amounted  to  209  per  cent,  of  that 
which  had  been  originally  applied.  That  the  nitrate 
had  been  reduced  to  gaseous  nitrogen  was  seen  by  the 
development  of  transverse  cracks  filled  with  gas  in  the 
soil,  and  it  was  concluded  that  some  of  the  nitrogen 
applied  in  manures  and  unaccounted  for  in  crop  and 
soil  may  well  be  due  to  the  reduction  of  nitrates  to 
gas,  by  the  combustion  of  organic  matter  with  the 
oxygen  of  the  nitrate,  especially  in  ill-drained  soils  in 
wet  weather.  Gayon  and  Dupetit,  in  1886,  isolated  two 
organisms  from  sewage  which  would  reduce  nitrates 
to  gas  in  the  presence  of  organic  matter,  the  action 
being  chiefly  carried  on  when  oxygen  was  absent ;  it 
came  to  a  standstill  when  plenty  of  air  was  supplied,  so 
that  the  organism  had  no  need  to  attack  the  nitrate 
to  obtain  oxygen.  Both  in  their  experiments  and  in 
others,  the  presence  of  an  abundant  supply  of  soluble 
organic  matter  was  one  of  the  necessary  conditions 
for  the  destruction  of  the  nitrates.  The  denitrifying 
bacteria  are  widely  distributed.  Warington  found,  out 
of  thirty-seven  species  of  bacteria  examined,  only 
fifteen  failed  to  reduce  nitrate,  twenty-two  reduced 
it  to  nitrite,  and  one  of  them  liberated  gas.  P.  F. 
Frankland,  again,  found  that  fifteen  out  of  thirty 
organisms  derived  from  dust  or  water  would  reduce 
nitrate  to  nitrite.  In  fact,  a  large  number  of  bacteria, 
when  deprived  of  oxygen  and  in  contact  with  abundant 


VII.] 


DEN1TR1FICA  T10N 


199 


organic  matter,  will  obtain  the  oxygen,  which  they 
normally  require  for  the  breaking  up  of  the  organic 
matter,  at  the  expense  of  the  nitrate. 

Many  experiments,  in  which  farmyard  and  other 
organic  manures  have  been  employed  in  conjunction 
with  nitrate  of  soda  and  similar  active  compounds  of 
nitrogen,  have  shown  a  smaller  crop  for  the  manures 
used  together  than  when  either  was  employed  singly. 
These  results  were  particularly  apparent  when  large 
quantities  of  material  like  fresh  horse-dung  or  chopped 
straw  were  used  in  pot  experiments.  With  well-rotted 
dung,  the  effect  of  organic  material  in  depressing  the 
yield  which  should  be  given  by  the  nitrate  was  not 
so  great 

The  nature  of  the  results  obtained  may  be  seen 
from  the  following  table,  which  gives  the  percentage 
recovered  in  the  crop  of  the  nitrogen  supplied  in  the 
manure,  when  used  alone,  or  in  conjunction  with  fresh 
horse-dung : — 

PERCENTAGE  OF  NITROGEN  RECOVERED  (WAGNER). 


Per  cent. 

When  used 

recovered  when 

with 

used  alone. 

Horse-dung. 

Nitrate  of  Soda     .... 

77 

52 

Sulphate  of  Ammonia   . 

69 

50 

60 

40 

Grass    

43 

:o 

Numbers  of  similar  experiments  in  pots  have  been 
recorded.  In  some  cases  the  use  of  fresh  dung  has 
even  resulted  in  a  smaller  crop  than  was  obtained  with- 
out any  manure  at  all ;  but  it  should  be  noted  that 
very  large  amounts  of  the  organic  manures  were  used, 


200      THE  LIVING  ORGANISMS  OF  THE  SOIL   [CHAP. 

equivalent  to  100  tons  or  more  per  acre.  Similar  results 
have,  however,  been  recorded  in  field  trials,  as  in  some 
experiments  of  Kriiger  and  Schneidewind's,  where  fresh 
cow-dung  was  applied  at  the  rate  of  23  tons  per  acre, 
horse-dung,  21  tons  per  acre,  and  wheat  straw  at  5«8 
tons  per  acre,  on  loth  July.  These  three  plots  were 
in  part  cross-dressed  with  urine  or  with  nitrate  of  soda, 
each  supplying  43  Ibs.  of  nitrogen  per  acre.  Two  suc- 
cessive crops  of  mustard  were  immediately  grown,  and 
the  amount  of  nitrogen  removed  by  the  crop  was 
ascertained.  Compared  with  the  wholly  unmanured 
plot,  the  cow-dung  alone  slightly  depressed  the  crop, 
about  1 1  Ibs.  per  acre  less  nitrogen  being  recovered; 
the  horse-dung  produced  a  depression  of  nearly  double 
this  amount ;  the  wheat  straw  produced  the  greatest 
depression,  its  crop  containing  about  18  Ibs.  per  acre 
less  nitrogen  than  that  given  by  the  unmanured  plot. 
Where  straw  was  used  with  nitrate  of  soda  the  two 
gave  a  crop  containing  23  Ibs.  less  nitrogen  per  acre 
than  the  nitrate  alone ;  where  urine  was  used  alone, 
the  produce  contained  25  Ibs.  more  nitrogen  per  acre 
than  when  it  was  used  in  conjunction  with  cow-dung 
and  straw. 

In  fine,  all  the  results  pointed  to  the  same  con- 
clusion— that  large  amounts  of  fresh  organic  manure 
not  only  do  not  themselves  help  the  crops,  but 
diminish  the  effect  of  other  rapidly  acting  nitrogenous 
manures  like  nitrate  of  soda,  sulphate  of  ammonia,  or 
urine. 

The  action  cannot,  in  the  two  latter  cases  at  least, 
be  put  down  to  denitrification  proper,  unless  it  is 
supposed  that  nitrification  and  subsequent  denitrifica- 
tion can  proceed  practically  simultaneously  in  the 
same  soil.  It  must  either  be  attributed  to  the  fact 
that  nitrification  is  very  much  checked  by  the 


vii.]  LOSSES  OF  SOIL  NITROGEN  201 

presence  of  large  amounts  of  organic  matter ;  or  to 
the  conversion  of  readily  available  nitrogen  into  a 
comparatively  insoluble  albuminoid  form  in  the  actual 
material  of  the  enormous  numbers  of  bacteria  that 
are  developed  by  the  free  food  supply ;  or,  lastly, 
to  those  fermentation  changes  of  organic  nitrogen  com- 
pounds which  result  in  the  liberation  of  free  nitrogen. 
Several  of  these  changes  may  take  place  together  ;  the 
essential  point  is,  that  nitrification  does  not  go  forward 
in  the  presence  of  much  organic  matter,  which  instead 
favours  all  the  bacterial  processes  resulting  in  the 
development  of  free  nitrogen. 

The  conditions  indeed  which  prevail  in  these  experi- 
ments are  scarcely  comparable  with  the  ordinary  practices 
of  agriculture.  Enormous  quantities  of  fresh  organic 
manure  are  employed  immediately  before  the  crop  is 
sown,  the  temperature  of  the  pots,  or  of  the  ground  in 
the  field  experiment  quoted,  is  very  high,  so  that  it 
is  easy  to  see  that  an  abnormal  condition,  both  as 
regards  nitrification  and  the  supply  of  oxygen  and  water, 
must  be  developed. 

There  are  not  lacking  both  long-continued  experi- 
ments and  ordinary  farming  experience  to  show  that 
nitrates  and  other  artificial  manures  can  be  used  in 
conjunction  with  dung  with  the  best  effects. 

For  example,  the  mangold  crop  at  Rothamsted 
shows  the  following  average  results  for  the  recovery  of 
nitrogen  from  various  nitrogenous  manures  used  first 
with  mineral  manures  alone  and  then  with  annual 
dressings  of  14  tons  per  acre  of  farmyard  manure,  a 
quantity  that  never  would  be  employed  so  frequently 
in  practice : — 


[TABLE. 


202      THE  LIVING  ORGANISMS  OF  THE  SOIL    [CHAP. 


NITROGEN. 

Nitrogenous  Dressing. 

YIELD. 
Tons 
per  acre. 

In 
Manure. 

Recovered 
in  Crop. 

Percentage 
recovered 
per  100  in 
Cross- 

dressing. 

PLOTS  MANURED  WITH  PHOSPHATES  AND  ALKALINE  SALTS. 

Nitrate  of  Soda    . 

17-95 

86 

67.2 

78-I 

Ammonium  Salts  . 

15-12 

86 

49-3 

57'3 

Rape  Cake   . 

20-95 

98 

69-4 

70.9 

Ammonium    Salts    and 

Rape  Cake 

24-91 

184 

103-0 

560 

PLOTS  MANURED  WITH  DUNG. 

Nothing 

17.44 

2OO 

63-3 

31-6* 

Nitrate  of  Soda     . 

24.74 

286 

115-8 

61-0 

Ammonium  Salts  . 

21-73 

286 

105-6 

49-2 

Rape  Cake   . 

23-96 

298 

in-i 

48-8 

Ammonium    Salts    and 

Rape  Cake 

24-05 

384 

129-8 

36-2 

*  Percentage  of  Nitrogen  in  dung  recovered. 

It  will  be  seen  that  all  the  nitrogenous  cross  dressings 
produce  an  increase  of  crop  when  added  to  the  farm- 
yard manure.  When  the  cross  dressings  are  used  on 
plots  receiving  only  non  -  nitrogenous  manures,  the 
nitrogen  recovered  varies  between  56  and  78  per  cent, 
of  that  supplied  in  the  manure ;  when  they  are  used 
in  conjunction  with  dung,  the  recovery  of  nitrogen  in 
the  increased  yield  above  that  produced  by  dung  alone 
varies  between  36  and  61  per  cent.  That  the  recovery  is 
smaller  in  the  latter  cases  is  due  to  the  fact  that  with  such 
excessive  amounts  the  yield  ceases  to  be  proportional 
to  the  supply  of  nitrogen,  being  limited  by  other  factors. 
Denitrification  is  only  likely  to  cause  rapid  loss 
of  nitrogen  when  large  quantities  of  nitrate  are 
applied  to  undrained  or  sour  land,  or  when  they  are 


vii.]  LOSSES  OF  SOIL  NITROGEN  203 

used  with  excessive  amounts  of  fresh  dung,  which  has 
not  been  rotted  and  so  deprived  of  much  of  its  soluble 
organic  matter.  Of  course,  a  steady  loss  of  nitrogen 
due  to  such  causes  as  have  been  enumerated  above  must 
also  be  expected  wherever  large  quantities  of  organic 
nitrogenous  manures  are  accumulating  in  the  land.  If, 
for  example,  we  compare  2  and  3  of  the  Broadbalk  wheat 
plots  at  Rothamsted,  the  latter  of  which  is  unmanured 
and  the  former  receives  dung  containing  200  Ibs.  of 
nitrogen  per  acre  every  year,  we  find  that  at  the  end 
of  the  fifty  years,  1844-93,  the  dunged  plot  contained 
in  the  top  18  inches  about  2680  Ibs.  more  nitrogen  than 
the  unmanured  plot,  or  a  mean  annual  accumulation  of 
50  Ibs.  The  extra  crop  grown  on  the  dunged  plot 
would  remove  a  further  31  Ibs.,  thus  leaving  115  Ibs. 
per  annum  to  be  accounted  for,  either  as  nitrogen 
washed  away  in  the  drainage  water  or  lost  as  gaseous 
nitrogen  by  denitrification  processes. 

Iron  Bacteria. 

Another  series  of  bacteria  playing  an  interesting 
part  in  certain  soils,  consists  of  those  which  secrete 
hydrated  ferric  oxide  or  bog-iron  ore  in  undrained  soils, 
where  the  soil  water  contains  ferrous  bicarbonate  in 
solution.  Winogradsky  investigated  four  of  these 
organisms,  to  whose  vital  processes  he  considered  the 
presence  of  soluble  ferrous  salts  was  essential.  Molisch, 
however,  regards  the  secretion  of  ferric  hydrate  as,  in 
a  sense,  an  accidental  accompaniment  of  their  growth, 
much  as  the  separation  of  large  quantities  of  silica,  so 
characteristic  of  the  straw  of  cereals,  is  unessential  to 
their  development.  It  has  already  been  noted  that 
these  iron  earths  do  not  form  in  soils  containing  calcium 
carbonate,  which  seems  to  prevent  the  formation  of  any 
soluble  ferrous  compounds. 


204      THE  LIVING  ORGANISMS  OF  THE  SOIL    [CHAP. 

Fungi  of  Importance  in  the  Soil. 

Allusion  has  already  been  made  to  the  fact  that  a 
large  number  of  fungi  inhabit  the  soil — Penicillium, 
Mucor,  Trichoderma,  Spicaria,  etc.,  Cladosporium,  Clado- 
thrix,  and  various  wild  yeasts,  Manilla,  etc.  —  all  of 
which  aid  in  breaking  down  the  organic  matter.  Many 
of  these  fungi  possess  the  power  of  attacking  ammonium 
salts  applied  as  manure,  withdrawing  the  ammonia  and 
setting  free  the  acid.  To  this  action  is  due  the  acidity 
produced  by  the  long-continued  use  of  ammonium  salts 
as  manure,  as  seen  on  the  experimental  plots  at  Rotham- 
sted  and  Woburn.  At  Woburn  the  soil  is  light  and 
sandy,  containing  but  little  lime,  and  the  application  of 
ammonium  salts  containing  50  Ibs.  ammonia  per  acre 
every  season  for  twenty-four  years,  has  rendered  the 
land  practically  incapable  of  carrying  the  crops.  A 
moderate  dressing  of  lime,  however,  restores  the  fertility. 
The  following  crops  were  obtained  in  1900  on  the  barley 
plots : — 


Ammonium  Salts 
only. 

Minerals 
+  Ammonium  Salts. 

With  no  Lime  .... 
With  2  tons  Lime,  applied  Nov. 
1897      

5-6 
28-9 

12-3 
33-7 

The  soil  had  become  acid  to  litmus  paper  where  the 
lime  had  not  been  used  :  it  is  interesting  to  note  that 
though  barley  would  not  grow,  oats  flourished  freely  on 
this  sour  soil.  There  are,  however,  two  special  organisms 
which  merit  further  consideration — the  fungus  which 
clothes  the  finer  rootlets  of  many  classes  of  plants, 
forming  mycorhiza  and  the  slime  fungus,  or  Plasmodio- 
phora  which  causes  the  disease  known  as  "finger-and- 
toe  "  or  "  club  "  in  turnips  and  other  cruciferous  plants. 


VII.]  MYCORHIZA  205 

The  term  "mycorhiza"  is  applied  to  the  symbiotic 
combination  of  the  filaments  of  certain  fungi,  whose 
complete  development  is  as  yet  unknown,  with  the 
finest  rootlets  of  certain  plants.  Sometimes  the  fungus 
forms  a  sort  of  cap  on  the  exterior  of  the  short  root- 
lets, which  are  generally  without  root  hairs ;  in  other 
cases  it  penetrates  the  cortical  tissue  of  the  root  itself, 
which  may  also  be  furnished  with  root  hairs.  Accord- 
ing to  the  researches  of  Frank,  the  fungus  of  the 
mycorhiza  lives  in  symbiosis  with  the  higher  plant, 
attacking  the  humus  and  also  the  mineral  resources  of 
the  soil,  and  passing  on  the  food  thus  obtained  to  the 
host  plant.  In  some  few  cases  the  host  plant  possesses 
no  green  assimilating  leaves,  and  is  wholly  dependent 
upon  mycorhiza  to  obtain  its  necessary  carbon  from 
the  humus.  Such  a  case  is  seen  in  the  Neottia  Nidus- 
avis,  or  Birds'  Nest  Orchis,  to  be  found  chiefly  amongst 
beech  underwood  in  this  country. 

More  generally,  the  host  plant  is  capable  of  nutri- 
tion in  the  ordinary  way  when  growing  in  media  in 
which  nutrient  salts  are  abundant,  but  becomes  myco- 
trophic  in  soils  and  situations  unfavourable  to  the  pro- 
duction of  directly  absorbable  food — as,  for  example,  in 
heaths  and  moors,  where  the  soil  is  almost  wholly 
humus,  or  beneath  the  shade  of  trees,  where  nitrates  are 
rarely  found  and  where  illumination  is  insufficient  for 
much  assimilation.  Later  researches,  particularly  those 
of  Stahl,  have  shown  that  the  symbiosis  of  mycorhiza, 
instead  of  being  a  phenomenon  restricted  to  a  few 
species,  is  widely  diffused  among  many  classes  of  plants, 
and  is  indeed  causally  connected  with  other  facts  of  wide 
general  importance  in  plant  nutrition.  It  has  already 
been  indicated  that  the  cultivated  plants  give  off  con- 
siderable quantities  of  water  by  transpiration  ;  the  form 
and  arrangement  of  their  leaves  are  adapted  to  expose 


206      THE  LIVING  ORGANISMS  OF  THE  SOIL    [CHAP. 

a  large  evaporating  surface,  the  root  is  well  developed 
and  provided  with  root  hairs  to  keep  up  the  supply  of 
water  to  the  plant.  There  are,  however,  a  number  of 
plants  in  which  transpiration  is  much  less  active,  and 
the  leaf  area  is  restricted  or  otherwise  arranged  to 
diminish  the  loss  of  water,  so  that  the  proportion 
previously  stated  as  existing  between  the  dry  matter 
produced  and  the  water  passing  through  the  plants 
is  greatly  diminished.  A  diminished  supply  of  water  to 
the  root  would,  however,  necessitate  a  loss  of  nutri- 
ment to  the  plant,  as  both  nitrates  and  other  mineral 
salts  enter  the  plant  with  the  transpiration  water. 
Stahl  has  shown  that,  in  general,  these  plants  with  a 
small  transpiration  activity  are  furnished  with  mycor- 
hiza,  by  means  of  which  they  obtain  food  of  all  kinds 
from  the  soil ;  whereas,  on  the  contrary,  the  plants, 
like  the  cereals,  the  cruciferous  and  leguminous  plants, 
Solanaceae,  etc.,  which  give  off  water  freely,  are  never 
associated  with  mycorhiza.  Many  of  the  conifers  and 
heaths  which  grow  on  dry  soils  show  this  correlation 
of  a  low  evaporation  and  restricted  leaf  development 
with  a  root-system  furnished  with  mycorhiza. 

Another  interesting  generalisation  has  also  been 
brought  into  line  with  the  above  facts  by  the  observa- 
tions of  Stahl  that  the  mycotrophic  plants  with  a 
feeble  transpiration  do  not  store  starch  in  their  leaves, 
but  contain  instead  considerable  quantities  of  soluble 
carbohydrates,  chiefly  glucose.  In  normal  plants,  though 
sugar  is  the  first  tangible  result  of  assimilation,  it  is 
rapidly  removed  from  the  sphere  of  action  by  being 
converted  into  starch,  such  withdrawal  of  the  product 
of  the  reaction  being  necessary  if  a  rapid  rate  of 
assimilation  is  to  be  maintained.  Should,  however, 
sugar  accumulate  in  the  cells,  the  concentration  of  the 
cell  sap  is  increased,  so  that  it  parts  with  its  water 


vii.]  MYCORHIZA  207 

by  transpiration  less  readily.  Though  many  excep- 
tions can  be  observed,  there  seems  to  be  a  very 
general  association  of  the  development  of  mycorhiza 
with  a  diminished  transpiration  and  the  absence  of 
starch  from  the  leaf,  especially  among  plants  like  the 
orchids,  lilies,  iris,  etc.,  which  often  grow  in  dry  or 
shady  situations,  such  plants  being  further  distinguish- 
able by  a  shiny,  glossy  leaf  surface.  Stahl  has  again 
shown  that  the  average  proportion  of  ash  to  dry 
matter  in  the  leaf  is  lower  for  mycotrophic  than  for 
normal  plants ;  the  former  grow,  as  a  rule,  in  situa- 
tions containing  but  little  mineral  salts,  particularly 
in  humic  soils,  where,  in  addition,  the  plant  is  put 
into  competition  for  whatever  nutriment  may  be  present 
with  the  mycelia  of  fungi,  which  everywhere  traverse 
humus  in  its  natural  state.  By  direct  experiment,  it 
has  been  shown  that  normal  plants  grown  in  humus 
develop  better  when  the  humus  is  previously  sterilised 
by  long  exposure  to  chloroform  vapour  than  when  it 
is  in  its  fresh  condition,  full  of  living  mycelia  com- 
peting successfully  for  the  nutriment.  The  absence 
in  the  leaf  of  calcium  oxalate  and  of  nitrates  is 
particularly  characteristic  of  mycotrophic  plants. 

Stahl  concludes  that  symbiosis  between  the  roots  of 
plants  and  the  mycelia  of  fungi  is  a  very  general 
phenomenon,  especially  characteristic  of  plants  growing 
in  soils  subject  to  drought,  or  poor  in  mineral  salts, 
or  rich  in  humus.  These  mycotrophic  plants  are 
generally  of  slow  growth,  possess  a  feeble  transpiration, 
and  limited  root  development ;  their  leaves  rarely  con- 
tain starch  ;  they  are  also  characterised  by  containing  a 
comparatively  small  proportion  of  mineral  salts,  among 
which  calcium  oxalate  and  nitrate  are  notably  absent. 

To  the  mycorhiza  associated  with  plants  of  the 
genus  Erica  the  power  of  fixing  atmospheric  nitrogen 


2o8      THE  LIVING  ORGANISMS  OF  THE  SOIL    [CHAP. 

has  been  attributed,  but  the  question  still  requires  further 
investigation. 

The  existence  of  mycotrophy  has  certain  interesting 
applications  in  practice ;  there  are  many  plants  which 
can  only  be  cultivated  with  difficulty  in  gardens ;  for 
example,  some  of  the  orchids,  ericas,  lilies,  and  others, 
generally  plants  which  must  be  grown  in  leaf-mould, 
peat,  or  other  material  rich  in  humus.  Yet  humus 
alone  is  not  always  sufficient  for  the  purpose,  the  peat 
or  leaf-mould  has  often  to  be  obtained  from  a  particular 
place ;  other  materials,  though  equally  rich  in  humus 
and  possessing  similar  mechanical  properties,  prove 
quite  unsuitable.  It  is  easy  to  surmise  that  this  effect, 
confined  in  the  main  to  mycotrophic  plants  and  humic 
soils,  may  easily  be  due  to  the  absence  of  the  proper 
fungus  from  the  soils  found  to  be  unsuitable. 

It  has  also  been  shown  that  the  difficulty  usually 
experienced  in  raising  seedlings  of  exotic  orchids, 
which  die  off  in  great  number  just  after  they  have 
germinated,  may,  to  a  large  extent,  be  obviated  by 
mixing  with  the  medium  in  which  the  seeds  are  sown 
a  little  of  the  material  in  which  the  parent  plants  are 
growing.  The  young  seedling  is  found  to  develop 
mycorhiza  at  a  very  early  stage,  and  then  only  will 
grow  properly. 

"  Finger-and-  Toe" 

On  many  soils,  particularly  those  of  a  sandy  nature, 
the  turnip  crop  is  often  almost  wholly  destroyed  by  the 
disease  known  as  "  finger-and-toe,"  "  club,"  or  "  anbury." 
Cabbages  and  other  cruciferous  crops  are  equally  at- 
tacked ;  so  much  so,  that  in  gardens  which  have  become 
infected  it  is  practically  impossible  to  raise  crops  of 
this  nature.  The  disease  is  caused  by  an  organism, 
Plasmodiophora  brassica,  belonging  to  the  slime  fungi, 


VII.] 


"  FJNGER-AND-TOE  " 


209 


and  forming  spores  which  may  remain  dormant  in  the 
soil  for  some  time,  certainly  for  two  or  three  years.  It 
has  long  been  known  that  the  best  remedy  against  finger- 
and-toe  consists  in  the  application  of  lime ;  and  as  far 
back  as  1859,  Voelcker  showed  that  soils  on  which  this 
disease  is  prevalent  are  deficient  in  lime ;  and  in  many 
cases  in  potash  also.  Later  researches  have  only  served 
to  emphasise  the  fact  that  the  disease  is  associated  with 
soils  of  an  acid  reaction,  in  which  calcium  carbonate  is 
wanting,  or  present  in  very  small  proportions.  The 
fungus,  as  is  generally  the  case  with  fungi,  refuses  to 
grow  in  a  neutral  or  slightly  alkaline  medium,  and  the 
only  way  to  get  rid  of  the  infection  in  the  land  is  to 
restore  its  neutrality  by  repeated  dressings  of  lime.  At 
the  same  time,  the  land  should  be  rested  as  long  as 
possible  from  cruciferous  crops  ;  uneaten  fragments  of 
diseased  turnips,  etc.,  should  not  be  allowed  to  go  into 
the  dung,  or  if  they  do,  the  dung  should  be  used  on 
the  grass  land.  Manures,  again,  which  remove  calcium 
carbonate  from  the  soil,  like  sulphate  of  ammonia,  or 
acid  manures  like  superphosphate,  should  not  be  em- 
ployed ;  neutral  or  basic  phosphates,  with  sulphate  of 
potash  on  sandy  soils,  should  be  employed  instead. 

The  following  figures  show  the  amount  of  lime  dis- 
solved by  hydrochloric  acid  from  soils  affected  with 
"  finger-and-toe,"  as  compared  with  spots  in  the  same 
field  where  the  disease  was  not  in  evidence  : — 


LIME  PKR  CENT. 

Voelcker. 

Voelcker. 

Hall. 

Sandy  Soil. 

Clay  Soil. 

Soils    affected 

by  disease  . 

•14 

•084 

•13 

•31 

•39 

Soils  free  from 

disease   .     . 

•89 

•52 

•43 

2io  THE  LIVING  ORGANISMS  OF  THE  SOIL  [CHAP.  VH. 

It  must  be  remembered  that  in  these  cases  the  total 
lime  soluble  in  acids  is  given,  not  merely  the  lime 
present  in  carbonate. 

Whenever  a  turnip  crop  is  seen  to  be  infected  with 
"  finger-and-toe  "  the  land  should  be  well  dressed  witht 
3  or  4  tons  per  acre  of  quicklime  immediately  'the 
crop  has  been  removed ;  as  long  an  interval  as  possible 
should  be  given  before  again  taking  a  cruciferous  crop, 
substituting,  for  example,  mangolds  for  turnips  in  the 
next  rotation ;  every  effort  should  be  made  to  destroy 
cruciferous  weeds  like  charlock ;  turnip-fed  dung  should 
not  be  applied,  and  another  dressing  of  finely  divided 
quicklime  should  be  put  on  for  the  crop  preceding 
the  sowing  of  the  new  turnip  crop. 


CHAPTER    VIII 

THE   POWER   OF  THE  SOIL  TO  ABSORB   SALTS 

Retention  of  Manures  by  the  Soil — The  Absorption  of  Ammonia 
and  its  Salts  ;  of  Potash  ;  of  Phosphoric  Acid — Chemical  and 
Physical  Agencies  at  Work — The  Non-Retention  of  Nitrates 
— The  Composition  of  Drainage  Waters — Loss  of  Nitrates 
by  the  Land — Time  of  Application  of  Manures. 

MANY  of  the  substances  employed  as  manures  are 
soluble  in  water,  hence  it  becomes  important  to  ascer- 
tain what  is  likely  to  be  their  fate  in  the  soil,  when 
their  application  is  followed  by  sufficient  rain  to  cause 
percolation  into  the  subsoil.  Of  substances  contain- 
ing nitrogen,  nitrate  of  soda,  the  salts  of  ammonia, 
urea,  and  kindred  bodies,  are  freely  soluble  in  water ; 
superphosphate  alone  of  the  compounds  of  phos- 
phoric acid  commonly  used  as  manure  is  soluble ;  but 
sulphate,  chloride,  and  carbonate  of  potash  are  easily 
soluble. 

Not  long  after  the  principles  underlying  the  nutri- 
tion of  plants  had  been  established,  Thompson  and  Way 
showed  that  ordinary  soil  possesses  the  power  of  with- 
drawing most  of  the  above  substances  from  solution, 
and  so  saving  them  from  washing  away  into  the  subsoil 
or  the  drains.  Some  of  the  salts,  like  sulphate  of 

ammonia,   are   decomposed,  the   base   alone  being  re- 
211 


212  POWER  OF  THE  SOIL  TO  ABSORB  SALTS  [CHAP. 

tained  and  the  acid  draining  through.  Way  found 
that  liquid  manure  from  a  dung-heap,  which  contains 
both  organic  and  ammoniacal  compounds  of  nitrogen, 
potash,  and  a  little  phosphoric  acid,  when  filtered  through 
a  short  column  of  soil,  parted  with  almost  the  whole  of 
its  organic  matter  and  much  of  its  salts  to  the  soil ; 
compounds  of  calcium  were,  however,  more  abundant  in 
the  filtered  liquid  than  before.  Way's  observations 
were  extended  by  Voelcker,  who  compared  the  absorb- 
ing powers  of  different  types  of  soils,  and  so  obtained  an 
idea  of  the  method  by  which  the  absorption  of  each 
substance  was  effected  ;  and  later  researches  have  only 
served  to  confirm  the  results  then  obtained.  It  was 
found  that  all  the  organic  compounds  of  nitrogen, 
ammonia — either  free  or  in  combination — phosphoric 
acid,  and  potash  were  wholly  removed  from  solution 
by  ordinary  soil,  though  some  soils  were  more  effective 
than  others ;  whereas  nitrates,  sulphates,  chlorides,  and, 
among  bases,  sodium  and  calcium,  were  only  slightly, 
if  at  all,  retained.  These  results  are  confirmed  by  the 
analysis  of  the  water  which  flows  from  land  drains 
under  normal  conditions ;  this  will  generally  be  found 
to  contain  nitrates  (sometimes  in  fair  quantity),  sulphates 
and  chlorides  of  calcium  and  sodium,  and  considerable 
amounts  of  calcium  bicarbonate,  but  rarely  shows  more 
than  a  trace  of  ammonia,  phosphoric  acid,  or  potash. 
The  absorptive  action  of  the  soil  is  partly  a  chemical 
process,  due  to  interactions  with  the  humus,  the  zeolitic 
double  silicates,  and  the  calcium  carbonate  of  the  soil ; 
and  partly  physical,  dependent  upon  the  extent  of 
surface  offered  by  the  soil  particles  (for  the  surface  of  a 
solid  possesses  the  power  of  concentrating  molecules  of 
any  dissolved  substance  in  the  layer  of  solution  with 
which  it  is  immediately  in  contact).  The  mechanism  of 
this  physical  "  adsorption  "  is  but  imperfectly  understood, 


VIIL]  ABSORPTION  OF  NITROGENOUS  COMPOUNDS  213 

but  even  pure  sand  will  remove  sodium  chloride  from 
solution  if  the  filtering  column  be  sufficiently  long;  it 
may,  again,  be  illustrated  by  the  phenomenon  of 
"laking,"  />.,  the  power  of  certain  colloid  bodies,  like 
the  hydrates  of  iron  and  alumina,  on  precipitation,  to 
drag  down  with  themselves  many  organic  substances 
from  solution. 

The  absorption  of  the  organic  compounds  of  nitrogen 
by  the  soil  seems  to  be  a  physical  process  of  this  kind, 
comparable  to  the  action  of  charcoal  in  absorbing 
ammonia  or  the  strongly  smelling  products  of  putre- 
faction, etc.  The  deodorising  powers  of  earth  for 
faecal  and  decomposing  matter  are  very  familiar ;  this 
means  that  fixation  in  a  more  or  less  insoluble  and 
non-volatile  state  of  various  organic  nitrogen  and  sulphur 
compounds  is  effected,  and  other  inodorous  nitrogen 
compounds  are  retained  in  the  same  way.  The 
absorption  is  most  marked  with  soils  rich  in  humus 
or  in  clay — the  soil  materials  which  present  the 
largest  surface.  The  absorptive  power  of  soil  for 
organic  compounds  of  nitrogen  is  well  seen  in  a 
sewage  farm,  the  object  of  which  is  to  so  far  purify 
sewage  by  percolation  through  a  few  feet  of  soil,  as 
to  fit  it  to  be  turned  into  a  river  without  danger  to 
health.  For  example,  on  the  Manchester  Sewage 
Works,  in  1900,  percolation  through  5  feet  of  soil 
reduced  the  organic  nitrogen  in  the  liquid  from  0-26  to 
0-056  parts  per  million,  and  the  free  ammonia  from  1-89 
to  0-92.  It  is  necessary,  also,  on  a  sewage  farm  to  work 
with  soils  possessing  but  a  small  absorbing  power  ;  only 
sandy  and  gravelly  soils  will  permit  of  rapid  enough 
percolation,  both  to  deal  with  large  volumes  of  sew- 
age and  afterwards  to  aerate  themselves  and  accom- 
plish the  destruction  by  bacterial  action  of  the  absorbed 
material.  Stiffer  soils  would  be  far  more  effectual 


214  POWER  OF  THE  SOIL  TO  ABSORB  SALTS  [CHAP. 

absorbents  of  the  sewage  material,  but  are  unsuitable 
because  they  do  not  admit  of  percolation. 

Absorption  of  Ammonium  Salts. 

The  absorption  of  free  ammonia  follows  the  lines 
indicated  above  for  the  absorption  of  the  organic  com- 
pounds of  nitrogen ;  but  the  salts  of  ammonia  are 
retained  by  the  soil  by  purely  chemical  processes  which 
result  in  the  formation  of  insoluble  salts  of  ammonia  in 
the  nature  of  double  silicates  and  humates.  Way  and 
Voelcker  first  found,  that  when  either  the  sulphate, 
chloride,  or  nitrate  of  ammonia  in  solution  is  allowed  to 
remain  in  contact  with  soil,  the  base  is  absorbed,  but  the 
acid  portion  of  the  salt  remains  in  solution  in  combina- 
tion with  lime.  Voelcker  also  showed  that  when  soil 
was  shaken  up  with  dilute  solutions  of  ammonium  salts, 
the  withdrawal  of  ammonia  from  the  solution  was  never 
complete,  but  varied  both  with  the  nature  of  the  soil  and 
the  strength  of  the  solution,  a  greater  proportion  being 
taken  from  weak  than  from  strong  solutions. 

The  absorption  of  the  ammonium  salts  by  the  soil  is 
now  known  to  be  due  to  the  combined  effects  of  at  least 
three  actions — upon  the  zeolites,  upon  the  humus,  and 
upon  calcium  carbonate.  With  the  zeolites  a  double 
decomposition  takes  place,  ammonium  becomes  insoluble, 
and  equivalent  amounts  of  calcium,  magnesium,  potas- 
sium (sodium  also  on  occasion)  enter  into  combination 
with  the  acid  in  the  solution,  for  no  acid  is  absorbed 
and  the  whole  solution  remains  neutral. 

The  reaction  is  a  reversible  one,  but  the  clay  con- 
taining the  zeolites  is  not  capable  of  absorbing  more 
than  a  certain  small  amount  of  ammonium  from  the 
strongest  solutions  of  its  salts.  The  following  table 
shows  the  ammonia  absorbed  by  100  grams  of  very 


VIII.] 


ABSORPTION  OF  AMMONIA 


215 


pure  clay   when   shaken   with   300  c.c.   of  ammonium 
chloride  solution  of  varying  strengths. 


Original 
Concentration  of 
Solution. 

Ammonia 
Withdrawn. 

Ammonia 
Withdrawn. 

Grams. 

Per  cent. 

N/io 

O-I26 

24-7 

N/I2 

0-115 

28>I 

N/iS 

0-104 

30-5 

N/20 

0-090 

35-3 

N/so 

0-068 

40-3 

N/5o 

0049 

48-0 

N/ioo 

0-03I 

600 

Thus  from  the  weaker  solutions  a  smaller  total  but  a 
larger  proportion  of  the  ammonia  was  removed  by  the 
clay,  and  the  removal  was  never  complete.  Of  course, 
in  the  field  the  amount  of  soil  is  so  enormously  in  excess 
that  the  absorption  of  ammonium  salts  applied  as 
manure  is  practically  complete.  Thus  at  Rothamsted 
the  presence  of  ammonia  in  the  drainage  water  is  rarely 
detected,  even  when  heavy  rain  immediately  follows  the 
application  of  the  manures. 

With  humus,  ammonium  salts  interact  in  a  very 
similar  fashion,  calcium  coming  into  solution,  and  the 
ammonium  forming  some  insoluble  compound  with  the 
complex  "  humic  "  acids. 

With  calcium  carbonate  a  double  decomposition  of 
the  type — 

(NH4)2SO4  +  CaCO3  ^I±  (NH4)2CO3  +  CaSO4 

— takes  place,  not  only  with  such  substances  as 
ammonium  sulphate,  but  also  with  humic  and  zeolitic 
compounds  of  ammonium,  and  though  the  proportion 
converted  into  carbonate  is  small,  it  is  constantly 
renewed  as  the  ammonium  carbonate  is  nitrified,  so  that 
eventually  the  whole  of  the  ammonium  salt  applied  to 
the  land  undergoes  this  change. 


216  POWER  OF  THE  SOIL  TO  ABSORB  SALTS  [CHAP. 

In  consequence,  the  continued  use  of  ammonium 
salts  as  a  fertiliser  results  in  the  depletion  of  the  stores 
of  calcium  carbonate  in  the  soil,  as  may  be  seen  from 
the  following  determinations  of  the  rate  of  disappearance 
between  1865  and  1904,  of  calcium  carbonate  from 
some  of  the  soils  of  the  Rothamsted  wheat  field  where 
the  calcium  carbonate  is  of  artificial  origin  and  is  con- 
fined to  the  surface  layer  of  the  soil. 


Bate  of  Loss 

Plot. 

Manuring. 

of 

Calcium  Carbonate. 

Lbs.  per  acre 

Per  acre  per  annum. 

per  annum. 

3 

Unmanured    

800 

5 

Mineral  Manures  only   

880 

6 

„             ,,          +  200  Ibs.  Ammonium  Salts 

1170 

7 

,(             „          +400         „                 „ 

IOIO 

8 

+  600 

1170 

9 

„             „          +412  Ibs.  Sodium  Nitrate  . 

565 

10 

400  Ibs.  Ammonium  Salts  only 

1045 

2 

Farmyard  Manure  

590 

Thus  the  use  of  ammonium  salts  increases  the 
normal  loss  of  calcium  carbonate  experienced  by  the 
soil  (due  to  solution  as  bicarbonate),  and  the  amount 
removed  increases  with  the  larger  applications  of 
ammonium  salts.  Taking  the  mean  of  these  and  other 
results  obtained  at  Rothamsted,  200  Ibs.  of  ammonium 
salts  causes  a  removal  of  about  120  Ibs.  calcium 
carbonate,  whereas  the  amount  calculated  from  the 
equation  given  above  would  be  about  160  Ibs.  That 
the  loss  from  the  plots  receiving  sodium  nitrate  and 
dung  is  less  than  from  the  unmanured  plot,  is  due,  in 
the  former  case,  to  the  base  left  in  the  soil  by  the  growth 
of  plants  which  derive  their  nitrogen  from  sodium  nitrate, 
and  in  the  latter,  to  calcium  carbonate  formed  by 
bacterial  action  from  organic  calcium  salts  in  the  dung. 


VIII.] 


POTASH 


217 


The  Absorption  of  Potash. 

In  all  respects  the  absorption  of  potash  follows  the 
same  laws  as  that  of  ammonia :  i.e.,  caustic  potash 
is  absorbed  directly,  but  sulphate,  nitrate,  and  chloride 
of  potash  undergo  a  double  decomposition,  by  which  the 
potash  is  retained  and  calcium  sulphate,  nitrate,  or 
chloride,  appear  in  the  water  draining  through  the  soil. 
Voelcker  found  in  laboratory  experiments  with  small 
quantities  of  soil  that  potassium  carbonate  was  more 
freely  absorbed  than  sulphate,  and  that  clays,  marls, 
and  pasture  soils  were  more  effective  in  retaining  potash 
than  light  loams  or  sands,  which  latter  had  but  little 
absorbing  power. 

The  following  table  shows  some  of  the  results 
obtained  when  potash  and  soda  salts  were  compared  : — 


Percentage  retained. 

Potash. 

Soda. 

Chalky  Loam 

3-6 

0-8 

Clay     . 

4-0 

I-I 

Sandy  Loam 

2-6 

0-6 

Pasture 

3-8 

10 

Loam    . 

3-4 

10 

Ironstone  Sand 

l-i 

0-6 

Both  the  humus  and  the  zeolitic  double  silicates  take 
part  in  the  retention  of  the  potash  salts,  the  reactions 
being  exactly  similar  to  those  taking  place  with  the 
ammonium  salts.  In  some  of  Way's  experiments  with 
pure  clays  the  application  of  potash  salts  was  followed 
by  the  appearance  of  the  corresponding  sodium  salts  in 
the  percolating  water,  though  with  most  soils  it  is 
calcium  that  is  turned  out  of  combination.  Potash  salts 
applied  to  the  soil  also  react  to  a  certain  extent  with  the 
calcium  carbonate,  giving  rise  to  a  little  potassium 
carbonate,  the  bad  effect  of  which  upon  the  tilth  of  the 


218  POWER  OF  THE  SOIL  TO  ABSORB  SALTS  [CHAP. 

soil  will  be  considered  later  (p.  253).  Dyer  has  examined 
the  soils  of  the  Rothamsted  wheat  plots  which  had  then 
been  continuously  manured  in  the  same  way  for  fifty 
years,  with  the  view  of  tracing  the  fate  of  the  mineral 
manures  applied.  The  following  table  shows  a  com- 
parison of  the  amounts  of  potash  soluble  in  strong  hydro- 
chloric acid,  in  Ibs.  per  acre,  found  in  the  top  9  inches 
of  soil  from  four  of  the  plots;  one  (No.  11)  received 
nitrogen  and  phosphates,  but  no  potash,  every  year,  the 
others  were  variously  manured,  but  all  received  200  Ibs. 
per  acre  of  sulphate  of  potash.  Estimates  are  also  given 
of  the  total  amount  of  potash  applied  as  manure  and 
removed  in  the  crops  over  the  whole  period,  so  that  in 
the  last  two  columns  a  comparison  can  be  made  between 
the  actual  surplus  of  potash  in  the  manured  over  the 
unmanured  soils,  and  the  surplus  calculated  from  the 
differences  between  the  potash  added  in  the  manure 
and  removed  in  the  crops  : — 


POTASH—  LBS.  PRR  ACRE. 

SURPLUS 
OVER  PLOT  11. 

Plot  and  Manuring, 

<»> 

a   . 

•0 

"8 

per  Acre. 

f|| 

«j 

1  1 

*  p" 

0    6 

£ 

g 

6 

•o 

I 

1  1,  receiving  no  Potash    . 

5107 

15 

1190 

7,  receiving  200  Ibs.  Sul- 

phate of  Potash 

6793 

5037 

2550 

3662 

1686 

5,  receiving  200  Ibs.  Sul- 

phate of  Potash 

7233 

5203 

1136 

5242 

2126 

13,  receiving  200  Ibs.  Sul- 

phate of  Potash 

7078 

5287 

2410 

4052 

1971 

On  the  whole,  about  one-half  of  the  estimated  surplus 
of  potash  received  by  the  manured  plots  still  remains  in 
the  top  9  inches  of  soil. 

Dyer  further  estimated  the  proportions  of  potash  in 
the  same  soils  which  was  soluble  in  a  r  per  cent,  solu- 


VIII.] 


PHOSPHORIC  ACID 


219 


tion  of  citric  acid,  and  found  that  both  the  surface  and 
the  subsoil  down  to  a  depth  of  27  inches  contained  more 
of  this  readily  soluble  potash  where  it  had  been  applied 
as  manure,  than  did  the  companion  plot  receiving  no 
potash,  as  will  be  seen  from  the  following  table  :— 


riot. 

LBS.  PKK  ACRK  OK  POTASH,  BOI.UUI.K 
IN  1  T'KR  CKNT.  ClTKIC  ACID. 

KrKiM.cs  OVKR  PLOT  11. 

First 
9  inches. 

Second 
9  inches 

Third 
9  inches. 

Calculated. 

Found. 

ii 
7 
5 
13 

83 
602 

799 
487 

75 
374 
598 

363 

101 
179 

257 
235 

3662 

5242 
4052 

896 

1395 
826 

These  determinations  show  that  soluble  potash  salts 
applied  to  the  land  are  retained  chiefly  by  the  surface 
soil,  as  much  as  one-half  of  the  estimated  additions  of 
potash  during  fifty  years'  manuring  being  found  there. 
Some  of  the  potash,  however,  sinks  further  and  is 
retained  in  the  subsoil ;  in  the  top  27  inches  a  large 
proportion — nearly  one-quarter  of  the  whole — remains 
in  such  a  loose  state  of  combination  that  it  is  soluble 
in  i  per  cent,  citric  acid,  and  so  may  be  regarded  as 
available  for  the  plant. 

Absorption  of  Phosphoric  Acid. 

The  retention  of  soluble  phosphoric  acid  by  the  soil 
is  more  easily  intelligible,  for  there  are  present  several 
substances  capable  of  forming  insoluble  compounds 
with  phosphoric  acid — e.g.,  calcium  carbonate,  hydrated 
ferric  oxide,  and  the  hydrated  silicates  of  alumina  which 
make  up  so  much  of  clay.  Sand  and  powdered  silicates 
like  felspar  have  been  found  to  possess  little  or  no 
power  of  removing  phosphoric  acid  from  solution,  nor 


220  POWER  OF  THE  SOIL  TO  ABSORB  SALTS  [CHAP. 

have   either  soil,  clay,  or  peat  which  have  been  pre- 
viously washed  with  hydrochloric  acid. 

The  following  table  shows  the  percentages  of  the 
total  phosphoric  acid  supplied,  which  were  removed 
from  solution  by  various  soils  after  remaining  in  con- 
tact for  the  specified  times,  the  ratio  between  soil  and 
phosphoric  acid  being  about  1000  to  i. 

PERCENTAGE  OF  PHOSPHORIC  ACID  ABSORBED  (VOELCKER). 


After  1  day. 

After  8  days. 

After  26  days. 

Red  Loam 

60 

78 

95 

Chalky  Soil    . 

89 

99 

100 

Stiff  Clay       . 

51 

62 

86 

Stiff  Subsoil   . 

48 

69 

74 

Light  Sandy  Soil 

53 

59 

73 

In  an  ordinary  soil  containing  a  sufficiency  of 
calcium  carbonate,  the  application  of  soluble  phosphoric 
acid  like  superphosphate  will  chiefly  result  in  the 
precipitation  of  di-calcium  or  "reverted"  phosphate, 
wherever  the  solution  meets  with  a  particle  of  calcium 
carbonate.  This  di-calcium  phosphate  is  a  compound 
easily  soluble  in  weak  organic  acids  or  in  water  con- 
taining carbonic  acid  :  hence  the  great  value  of  applica- 
tions of  superphosphate  on  soils  rich  in  lime,  for  thus  a 
readily  available  phosphate  is  very  quickly  disseminated 
throughout  the  ground  in  a  state  of  fine  division.  But 
on  soils  poor  in  calcium  carbonate  the  precipitation  will 
be  chiefly  effected  by  the  hydrated  iron  and  aluminium 
compounds,  and  the  resulting  phosphates  are  practically 
insoluble  in  water  containing  carbonic  acid,  and  but 
little  in  saline  solutions  or  in  weak  organic  acids. 
Hence  applications  of  superphosphate  to  such  soils 
become  much  less  available  to  the  crop,  and  should  be 
preceded  by  a  thorough  liming  of  the  land.  Even  a 


viii.]  PHOSPHORIC  ACID  221 

subsequent  liming  on  soils  containing  phosphates  of 
iron  or  alumina  will  help  to  bring  them  into  a  more 
available  form,  because  a  double  decomposition  result- 
ing in  calcium  phosphate  and  aluminium  or  ferric 
hydrate,  will  proceed  to  an  extent  dependent  on  the 
mass  of  lime  present  in  the  medium. 

Further  evidence  of  the  precipitation  of  phosphoric 
acid  within  the  soil  is  afforded  by  Dyer's  examination 
of  the  Rothamsted  wheat  soils  at  various  depths,  after 
fifty  years'  continuous  manuring  with  and  without  super- 
phosphate. By  comparing  the  amount  of  phosphoric 
acid  contained  in  the  soil  of  the  unmanured  plot 
with  that  contained  in  the  soils  of  the  plots  receiv- 
ing superphosphate  every  year,  and  knowing  also  the 
amount  removed  by  the  successive  crops  in  each  case, 
it  is  possible  to  calculate  the  surplus  that  should 
remain  in  the  manured  over  the  unmanured  plots,  on 
the  assumption  that  the  soil  was  uniform  at  starting. 
Calculating  in  this  way,  Dyer  found  that  no  less  than 
83  per  cent,  of  the  phosphoric  acid  which  six  of  the 
plots  should  possess  after  fifty  years'  manuring  was 
still  present  in  the  top  9  inches  of  soil,  whereas  the 
subsoils  from  9  inches  to  18  inches,  and  18  inches  to 
27  inches,  showed  no  accumulation  of  phosphates. 
Dyer  further  determined  the  phosphoric  acid  which 
was  soluble  in  a  i  per  cent,  solution  of  citric  acid, 
and  found  that  on  the  manured  plots  the  top  9  inches 
of  soil  contained  about  39  per  cent,  of  the  estimated 
surplus  of  phosphoric  acid  so  combined  as  to  be 
soluble  in  this  medium,  whereas  in  the  subsoils  the 
"  available "  phosphoric  acid  was,  if  anything,  less  for 
the  manured  than  for  the  unmanured  plots.  It  has 
already  been  pointed  out  (p.  163)  that  if  the  extraction 
with  citric  acid  be  repeated,  practically  the  whole  of  the 
phosphoric  acid  applied  as  manure  and  not  removed  in 


222  POWER  OF  THE  SOIL  TO  ABSORB  SALTS  [CHAP. 

the  crop  can  be  recovered  from  the  top  9  inches  of  these 
Rothamsted  soils.  It  is  clear,  then,  that  soils  well 
provided  with  calcium  carbonate,  as  the  Rothamsted  soil 
is,  will  precipitate  very  near  the  surface  any  soluble 
phosphoric  acid  applied,  and  retain  it  for  a  long  time  in 
a  form  easily  redissolved  and  obtainable  by  the  plant. 
It  follows,  therefore,  that  superphosphate,  the  most 
soluble  of  the  phosphatic  manures,  can  be  applied  to 
normal  soils  in  the  winter  or  early  spring  without  any 
fear  of  the  phosphoric  acid  being  washed  out. 

The  Composition  of  Drainage   Waters. 

Further  evidence  of  the  fate  of  the  various  substances 
applied  as  manures,  their  retention  or  otherwise  by  the 
soil,  can  be  obtained  by  studying  the  composition  of  the 
water  flowing  from  land  drains. 

The  drainage  from  the  continuously  manured  wheat 
plots  at  Rothamsted,  each  of  which  possesses  a  tile  drain 
running  down  the  centre  at  a  depth  of  2  feet  to  2  feet 
6  inches,  has  been  collected  from  time  to  time  and  com- 
pletely analysed  by  Voelcker  and  Frankland  ;  in  addition, 
systematic  determinations  of  the  nitrogen  contents  have 
been  made  for  many  years.  In  a  general  way,  the  chief 
constituent  of  the  various  drainage  waters  is  lime,  either 
as  bicarbonate,  sulphate,  chloride,  or  nitrate  ;  soda  is 
the  only  other  base  present  in  any  quantity,  very  small 
amounts  of  magnesia,  potash,  and  ammonia  pass  into 
the  drains.  Of  the  acid  radicles,  chlorine  and 
sulphuric  acid  predominate  according  to  the  manuring, 
and  the  proportion  of  phosphoric  acid  is  minute ;  but 
the  amount  of  nitric  acid  varies  according  to  the 
manure  applied  and  the  season  at  which  the  water  is 
collected. 

The  following  table  shows  the  complete  analysis  of 
the  drainage  water  from  twelve  of  the  plots  : — 


VIM.]        ROTHAMSTED  DRAINAGE  WATERS 


223 


s 

CO 

SSBf 

w>  N^         M   1-4                     »o  C7^        w^  ^ 

1 

I 

j! 

\O                                             O^ 
to  ^  •-<   ^  fi  to  M   r^vO   C^  O   ^   »*> 

ro 

a 

oo 
g 

£•       ei»ud[ng+ 
8 
JO 

*^>  t**»  o  ^*  ^^  covo  ro^o  ^O  **  ^*  oo 

•^-  «           O                               **>00           *^  M 
<S 

1 

M 
O 

fr 

O                                           sO 

Os 

a 

X 

a        »l«qdiiig+ 

-"°^N-"°^M^" 

O 

CO 

a 

g 

0 

c<>  —«         \O                               tO  u^          ^-  M 

vr> 

vO   Osf   «^-Os«99^^^-Os 

O> 

s 

0 

ro  1-1         vn                          CO  ^        t>  M 

t 

I^I-N".-.  Os«  M  «  9  9        90 

9s 

o» 

rl    —  '          —                I/)         —  .    --     •    [  ^  ^ 

5 

^Oo'^eoONaNMi-.^^OQ 

^. 

t- 

3 

•sqt  oot  + 

fO    •-<              OO                          I"1                M      OS             UO    HH 

Os 

•*• 

o  o                              •<*- 

VO 

w 

* 

•sqi  OOS+ 

M              ^-             <->         t*  t-*       *r>  N 

s 

O  to                                         1-1 

9 

10 

0 

Os^O^-vOvn^.Tt-.-voO^:^ 

CO 

^ 

O   r<                                                **i 

^- 

co 

0 

W                O**                            **    W          ^*  ^^ 

M 

bo 

M      H^      HH      Tj-    tj\    Tj*   t-^'sD      t^»    ^                <O    t**» 

„ 

« 

a 

W      HH                *^-                         —  >                W      O          "      ^4"    f> 

•<*• 

60 

a 

"o  ,g 

0    |       

"o 

C/3 

a 
e 

M 

iftifijlijjj 

0 

H 

224    POWER  OF  THE  SOIL  TO  ABSORB  SALTS  [CHAP. 

An  examination  of  these  figures  shows  that  the 
amount  of  organic  matter  and  ammonia  reaching  the 
drains  is  practically  nil;  the  organic  matter  supplied 
as  dung,  and  the  ammonia,  which  is  employed  up  to 
400  Ibs.  of  mixed  ammonium  chloride  and  sulphate  per 
acre,  are  wholly  retained  by  the  soil.  The  effect,  how- 
ever, of  adding  either  organic  compounds  of  nitrogen  or 
ammonium  salts  is  to  increase  the  proportion  of  nitrates 
in  the  drainage  water.  Lime  is  the  chief  constituent  of 
the  dissolved  matter  in  the  drainage  waters,  the  propor- 
tion is  lowest  for  the  unmanured  plot  (3),  it  rises  with 
the  application  of  minerals  (5),  and  rises  again  with -each 
successive  application  of  ammonium  salts  (6)  and  (7).  The 
formation  of  calcium  chloride  and  sulphate  respectively, 
when  the  corresponding  ammonium  salts  are  applied  to 
land  containing  calcium  carbonate,  has  already  been 
discussed  :  it  is  well  seen  in  the  increased  richness  in 
lime,  sulphuric  acid,  and  chlorine  of  the  drainage  water 
from  6  and  7,  which  receive  200  and  400  Ibs.  respectively 
of  ammonium  salts,  as  compared  with  5,  which  receives 
the  same  minerals  without  any  nitrogen  compounds.  Plot 
1 1  receives  superphosphate  in  addition  to  the  ammonium 
salts  which  10  receives:  the  effect  of  the  gypsum  con- 
tained in  the  superphosphate  is  seen  in  the  increased 
lime  and  sulphuric  acid  content  of  the  drainage  water  of 
1 1 .  The  increase  is  not  so  great,  however,  as  that  caused 
by  the  addition  of  sulphates  of  potash  and  magnesia  to 
the  superphosphate  and  ammonium  salts  (plots  1 3  and  14), 
whereas  sulphate  of  soda  causes  little  loss  of  lime  (12). 
The  use  of  nitrate  of  soda  on  plot  9  causes  no  increase 
in  the  proportion  of  lime  in  the  drainage  water,  but  a 
large  quantity  is  removed,  chiefly  as  sulphate,  from  plot 
2,  receiving  dung  every  year.  The  quantity  of  lime 
removed  annually  in  this  way  will  be  very  great : 
assuming  a  mean  annual  drainage  equal  to  10  inches 


viii.]  LOSSES  TO  SOIL  IN  DRAINAGE  WATERS     225 

of  water,  the  unmanured  plot  will  lose  about  220  Ibs. 
per  acre  per  annum  of  lime  :  equivalent  to  about  400  Ibs. 
of  carbonate  of  lime,  whereas  the  analysis  of  the  soil 
shows  (p.  216)  an  annual  loss  of  about  800  Ibs.  per  acre. 
The  discrepancy  between  these  two  figures  is  due  to  the 
fact  that  the  results  are  calculated  from  but  a  small 
number  of  analyses  of  the  drainage  water,  the  amount 
of  which  is  also  very  uncertain.  When  400  Ibs.  of 
ammonium  salts  are  used  as  manure,  either  alone  or 
with  minerals,  the  increased  loss  of  lime  calculated  on 
the  same  basis  amounts  to  126  Ibs.  or  225  Ibs.  of 
carbonate  of  lime  per  acre  per  annum,  as  against  about 
240  Ibs.  found  from  the  analysis  of  the  soil. 

The  amount  of  magnesia  lost  is  small,  5  to  20  Ibs. 
per  acre  per  annum,  nor  is  the  amount  reaching  the 
drainage  water  much  increased  by  its  application  as 
manure  to  plots  5,  6,  7,  9,  and  14. 

The  amount  of  potash  lost  is  still  smaller,  from 
3  to  12  Ibs.  per  acre  per  annum,  but  it  is  distinctly 
dependent  on  the  amount  supplied  as  manure,  being 
at  a  maximum  with  the  dunged  plot  (2)  and  the  plot 
receiving  minerals  only  (5),  and  greater  from  all  the 
other  plots  receiving  potash  than  from  those  without 
it,  i.e.,  3,  10,  u,  12,  14.  The  use  of  sulphate  or 
nitrate  of  soda  increases  the  amount  of  potash  in  the 
drainage  water,  not  so,  however,  the  use  of  sulphate 
of  magnesia.  Practically  all  the  soda,  chlorine,  and 
nearly  all  the  sulphuric  acid,  that  are  applied  in  the 
manure  pass  through  into  the  drainage  water. 

A  comparison  of  the  drainage  waters  in  winter 
and  spring  shows  that  they  are  more  concentrated 
in  the  winter,  because  the  manures  (excepting  the 
nitrate  of  soda)  have  then  been  recently  applied :  the 
chlorides  wash  out  first,  then  the  sulphates,  and  as 
the  season  advances  not  only  is  the  total  amount  of 

P 


226   POWER  OF  THE  SOIL  TO  ABSORB  SALTS  [CHAP. 

lime  present  much  diminished,  but  it  comes  away 
chiefly  as  carbonate.  With  the  growth  of  the  crop 
in  spring  the  nitrates  disappear  from  the  drainage 
waters. 

The  amount  of  nitrates  found  in  the  drainage  water 
varies  not  only  with  the  time  of  year,  but  also  according 
to  the  interaction  of  temperature,  growth  of  crop,  culti- 
vation, and  percolation.  Nitrates  are  only  rapidly  pro- 
duced when  the  temperature  of  the  soil  has  risen  :  if  the 
percolation  is  not  excessive  the  crop  may  remove  the 
nitrates  as  fast  as  they  are  formed,  but  a  heavy  rainfall 
in  the  spring  before  the  nitrates  have  been  much  drawn 
upon  by  the  crop,  or  one  just  after  the  land  has  been 
broken  up  in  the  autumn  and  is  still  warm,  will  result  in 
a  considerable  washing  out  of  nitrates.  At  the  same 
time  a  certain  amount  of  moisture  in  the  soil  is  necessary 
for  the  formation  of  nitrates,  and  the  crop  itself  may  so 
dry  the  soil  as  to  reduce  nitrification  considerably.  The 
following  table  (p.  227)  shows  the  estimated  loss  of 
nitrates  from  the  same  wheat  plots  at  Rothamsted  as  have 
previously  been  dealt  with,  during  two  years,  each  of 
which  has  been  divided  into  two  periods :  firstly,  from 
the  date  at  which  the  nitrogenous  manures  were  sov/n  up 
to  harvest ;  and  secondly,  from  harvest  round  again  to 
the  sowing  of  manures  in  spring. 

The  diagram  (Fig.  15)  shows  the  same  results  in  a 
graphic  form. 

The  seasons  were  rather  exceptional,  the  summer 
rainfall  and  drainage  in  1879  and  the  winter  rainfall  in 
the  following  year  being  both  above  the  average.  It 
will  be  seen  that  except  on  the  autumn  manured  plot 
15,  the  loss  was  greatest  from  plot  9  receiving  55olbs. 
of  nitrate  of  soda,  and  this  excess  of  loss  was  chiefly  in 
the  summer  drainage  water  of  1879;  the  figures  are, 
however,  exaggerated  by  the  fact  that  half  the  nitrate 


i bs.  ol  .Nitrogen  p«r  acre,         iu  ^u  3U  40 

FIG.  15. — Losses  of  Nitrogen  in  Drainage  from  Rothamsted  Wheat  Plots. 

Black  =  Losses  in  Summers  1S79,  1SSO. 
Shaded  ^  Losses  in  following  Winters. 


[  To  /iic(  page  2-t> 


VIII.] 


NITRATES  IN  DRAINAGE  WATER 


227 


plot  received  no  mineral  manures,  and  therefore  grew 
but  a  scanty  crop.  The  losses  during  the  winter  months 
are  more  nearly  the  same  for  all  plots,  and  represent  to 
a  large  degree  the  nitrification  of  the  organic  residues  in 
the  soil.  The  losses  from  the  plots  receiving  minerals 
and  varying  amounts  of  ammonium  salts  (5,  6,  and  7) 
increase  with  each  application  of  nitrogen :  the  losses 

NITRIC  NITROGEN  IN  DRAINAGE  WATER. — LBS.  PER  ACRE. 


1879-80. 

1880-81. 

to 

3% 

3  • 

<?£ 

Plot. 

Manuring,  per  Acre. 

fl 

«* 

11 

^ 

&t 

SoB 

o?£; 

1« 

g£    •» 

E  t« 

)Kf    ** 

8  ° 

is 

a~ 
0. 

Us 

ao. 

aa 

00 

CO 

CO 

•3 

T  .7 

IO-8 

0-6 

J 

5 

Minerals  only  

*  / 
1-6 

13-3 

o-7 

17.7 

6 

Minerals  +  200  Ibs.  Ammonium  Salts 

IO-I 

12-6 

2-2 

19-8 

7 

,,       +400     „ 

18-3 

12-6 

4-3 

21-4 

9 

,.       +S5O     >.     Nitrate  of  Soda 

45-0 

15.6 

15-0 

10 

400  Ibs.  Ammonium  Salts  alone 

42-9 

7-4 

35-2 

ii 

Do.           do.        +  Superphosphate 

28-3 

17.7 

3-4 

29-6 

12 

Do.           do.        +Sulph.  Soda 

21-2 

17-5 

3-3 

27-2 

13 

Do.           do.        +Sulph.  Potash   . 

I9O 

1  6-4 

3-7 

25-3 

14 

Do.          do.       +  Sulph.  Mag.     . 

26-O 

16-8 

4-2 

25-9 

15 

Minerals  +  400  Ibs.  Ammonium  Salts 

0  6 

Cn.o 

3»A 

1  A  .n 

by  y 

4 

74  y 

Estimated  Drainage  —  inches 

II-I 

4-7 

1-8 

18-8 

from  the  plots  receiving  ammonia  and  various  mineral 
manures  diminish  as  the  mineral  manure  becomes  a 
more  complete  plant  food,  because  the  greater  growth  of 
crop  thus  secured  more  completely  removes  the  nitrates 
as  they  are  formed,  besides  hindering  nitrification  by 
drying  the  surface  soil. 

The    effect    on    nitrification    of   crop    and    surface 
cultivation  is  well  seen  in  the  following  table  of  results 


228    POWER  OF  THE  SOIL  TO  ABSORB  SALTS  [CHAP. 

obtained  by  Deherain,  who  collected  the  drainage  from 
cement  tanks  2  m.  cube  and  systematically  filled  with 
soil  taken  from  corresponding  depths  in  the  field.  The 
soils  had  been  several  years  in  the  tanks,  so  that  they 
had  settled  down  into  practically  normal  conditions, 
though  the  effect  of  the  aeration  and  disturbance  of 
the  soil  in  filling  the  tanks  is  still  visible  in  a  rather 
high  rate  of  nitrification.  Each  tank  carried  the  crop 
indicated  in  the  first  column. 


Cropping. 

Drainage. 

Nitrogen  as  Nitric 
Acid. 

Fallow,  no  cultivation      . 
Rye  Grass       
Oats        

Inches. 
II-2 

7-8 

7-3 
6-0 

Lbs.  per  acre  in 
Drainage  Water. 

186-70 
2-28 

7-37 
2  1  -60 

Wheat,  followed  by  Vetches    . 
Wheat     . 

6-6 

7.c 

12-90 
28-70 

II.  e 

TQ6.c6 

Fallow,  no  cultivation 
Fallow,  hoed  and  rolled  . 
Vine        

1  1  -2 
1  1  -2 

•J.C. 

158-00 
183-20 
36-20 

7-2 

O.27 

The  rainfall  of  the  year  in  question,  March  1896  to 
March  1897,  amounted  to  28*8  inches,  most  of  which 
fell  in  the  autumn.  The  most  noteworthy  results  are 
the  effect  of  the  various  crops  in  diminishing  the  loss 
of  nitrates,  which  is  not  wholly  to  be  attributed  to  the 
quantity  taken  up  by  the  crop,  because  the  sum  of  the 
nitrogen  removed  in  the  crop  and  that  carried  off  in 
the  drainage  water  is  never  equal  to  the  nitrogen 
removed  from  the  uncropped  plots  by  the  drainage 
water  alone.  During  the  comparatively  dry  spring 
months  the  crops  leave  so  little  moisture  in  the  soil  that 
nitrification  is  checked,  and  the  total  production  of 


VIM.]      TIME  OF  APPLICATION  OF  MANURES        229 

nitrates  is  less  where  there  is  a  crop  than  on  the  moister 
uncropped  plots. 

When  the  wheat  was  followed  by  a  crop  of  vetches 
the  loss  of  nitrates  during  the  comparatively  wet  autumn 
was  considerably  reduced.  Lastly,  the  hoeing  of  the 
fallow  plots  resulted  in  a  considerably  increased  produc- 
tion of  nitrates. 

Time  of  Application  of  Manures. 

The  facts  set  out  above  as  to  the  retention  of  most 
of  the  soluble  constituents  of  manures  by  the  soil,  while 
the  nitrates  are  liable  to  wash  out,  have  an  impor- 
tant bearing  on  the  season  at  which  artificial  manures 
should  be  sown.  In  the  first  place  it  is  evident  that 
there  is  no  danger  of  losing  phosphates,  or  even  of 
their  washing  deep  into  the  soil,  when  employed  in 
their  most  soluble  form  as  superphosphate.  It  is  the 
general  custom  to  sow  superphosphate  with  the  drill 
for  roots  at  the  same  time  as  the  seed :  the  large 
quantity  of  manure  near  the  seedling  in  its  early  critical 
stages  is  probably  valuable,  and  as  the  roots  of  swedes 
and  turnips  do  not  extend  very  deeply,  the  phosphoric 
acid  may  be  placed  in  the  most  likely  place  to  reach 
them. 

But  for  more  deeply  rooting  crops,  hops  and  fruit 
or  even  mangolds,  it  seems  probable  that  superphos- 
phate is  often  applied  rather  too  late  in  the  season, 
and  that  if  used  as  a  winter  instead  of  a  spring  dressing 
it  would  have  a  better  chance  of  getting  well  diffused 
through  the  soil.  Basic  slag  and  other  insoluble  phos- 
phates should  be  used  in  the  winter  or  even  the  autumn  : 
there  is  no  risk  of  loss,  and  as  much  rain  as  possible 
is  wanted  to  get  them  distributed  in  the  soil.  As 
regards  potash  salts,  Dyer's  experiments  go  to  show 
that  they  descend  further  in  the  soil,  and  are  a  little 


230   POWER  OF  THE  SOIL  TO  ABSORB  SALTS  [CHAP. 

more  subject  to  washing  than  the  soluble  phosphates : 
for  this  reason,  where  sulphate  of  potash  is  employed, 
as  for  potatoes,  it  will  best  be  sown  with  the  seed. 
Where  kainit  is  used,  it  is  best  employed  as  a  winter 
or  autumn  dressing ;  there  will  be  little  loss  of  potash, 
for  this  will  get  fixed  chiefly  in  the  surface  soil.  But 
the  chlorides,  which  are  present  in  kainit  and  are 
sometimes  not  wholly  beneficial  in  their  action  upon  the 
crop,  will  be  removed  and  washed  out  into  the  drains  or 
the  subsoil  water  by  the  winter  rains :  the  magnesia 
salts  also  will  be  precipitated  within  the  soil,  and  to  a 
large  extent  removed  from  possible  action  upon  the 
crop.  Turning  to  the  nitrogen  compounds,  it  is 
necessary  to  keep  in  mind  that  all  of  them  will  become 
transformed  into  nitrates  which  are  liable  to  be  washed 
out.  All  insoluble  organic  manures  should  be  put 
on  before  or  during  the  winter :  the  decay  processes 
will  begin,  resulting  in  the  formation  of  amino-acids, 
ammonia,  etc.,  which  will  become  fixed  in  the  soil,  but 
the  low  winter  temperatures  will  not  permit  of  much 
nitrification.  Liquid  manure  and  similar  materials 
containing  such  readily  nitrifiable  substances  as  urea 
and  ammonium  carbonate,  should  be  reserved  until 
early  spring,  so  that  the  crop  may  be  growing  whenever 
nitrification  begins.  Ammonium  salts  are  very  rapidly 
nitrified,  so  that  they  should  only  be  used  in  spring. 
At  Rothamsted  nitrates  begin  to  appear  in  the  drainage 
water  immediately  after  the  application  of  the  ammonium 
salts  to  the  wheat  plots  in  March  if  rain  falls,  and  one 
of  the  plots  which  has  its  ammonium  salts  applied  in 
autumn  shows  not  only  a  considerable  falling  off  in 
crop  but  also  large  quantities  of  nitrates  in  the  winter 
drainage  waters. 

The  following  table  shows  the  amounts  of  nitrates 
removed  in  the  drainage  water  from  the  two  plots  which 


VIII.] 


NITRATES  IN  DRAINAGE  WATERS 


23' 


receive  400  Ibs.  per  acre  of  ammonium  salts  with  mineral 
manures,  only  differing  in  the  fact  that  on  plot  7  the 
ammonium  salts  are  sown  in  March,  and  on  plot  15  in 
October ;  also  the  average  crops  of  grain  and  straw  for 
the  twenty-three  years,  1875-97. 

1879-1881.— LBS.  OF  NITRIC  NITROGEN  PER  ACRE. 


Plot. 

1879-80. 

1880-81. 

CROP. 

Spring  Sowing 
to  Harvest. 

Harvest  to 

Spring  Sowing. 

Spring  Sowing 
to  Harvest. 

Harvest  to 
Spring  Sowing. 

Grain. 

Straw. 

Bushels. 

Cwt. 

7 
15 

I8-3I 
9-62 

12-63 
59-92 

4-29 

21-38 
74-94 

3iJ 

28$ 

31 
26^ 

Drainage 

II-l" 

4-7" 

,.S" 

1  8-8" 

That  sulphate  of  ammonia  will  to  some  extent 
persist  in  the  soil,  and  become  available  for  a  succeed- 
ing crop,  after  even  a  whole  year  has  elapsed,  is  to  be 
seen  from  the  results  of  the  Woburn  experiments  upon 
wheat.  Some  of  the  plots  at  Woburn  receive  mineral 
manures  every  year,  but  ammonium  salts  or  nitrate  of 
soda  only  every  alternate  year :  in  both  cases  the  crop 
falls  very  much  in  the  years  of  no  nitrogen,  but  the 
decrease  is  by  no  means  so  marked  with  ammonium  salts 
as  with  nitrate  of  soda,  which  latter  seems  to  leave  no 
residue  whatever. 

The  soil  at  Woburn  is  an  open  sandy  loam  ;  but  in 
the  years  for  which  the  results  are  quoted  (see  Table, 
p.  232)  the  rainfall  was  low. 

The  difference  between  the  results  set  out  in  the 
above-mentioned  table  and  those  obtained  upon  the 
corresponding  plot  at  Rothamsted,  where  the  dressing 


232    POWER  OF  SOIL  TO  ABSORB  SALTS    [CHAP.  vm. 

of  ammonium  salts  every  other  year  seems  to  leave  no 
residue  for  the  following  year,  may  perhaps  be  set  down 
to  the  different  texture  of  the  two  soils.  The  ammonium 
salts  are  converted  which  are  washed  down  into  the 
subsoil ;  at  Woburn  they  can  rise  again  by  capillarity, 
as  the  soil,  though  sandy,  is  still  fine  in  texture ;  at 
Rothamsted  the  soil  is  too  close-grained  to  admit  of 
any  considerable  movement  of  the  subsoil  water  back 
to  the  surface. 


Plot. 

1899. 

Bushels. 

1900. 

Bushels. 

18B 

Minerals  only    . 

20-3 

Minerals  +  Ammonium 

8A 

Minerals  +  Ammonium 

Salts   . 

27-3 

Salts   . 

33-1 

Minerals  only   . 

16-2 

9B 

Minerals  only    . 

9-8 

Minerals  +  Nitrate  of 

9A 

Minerals  +  Nitrate  of 

Soda    . 

27-7 

Soda   . 

41 

Minerals  only   . 

6-8 

4 

Minerals  only,  every 

year     . 

6-9 

Minerals  only   . 

5-9 

The  experiments  recorded  above  and  the  results 
of  the  examination  of  drainage  waters  go  to  show 
that  nitrate  of  soda  should  only  be  employed  when 
there  is  a  crop  in  possession  of  the  ground  and  ready 
to  seize  upon  the  salt  as  soon  as  it  becomes  diffused 
through  the  soil.  Only  on  dry  soils  can  it  be  safely 
applied  as  early  as  the  sowing  of  the  seed ;  in  wet 
climates  sulphate  of  ammonia  will  often  be  preferable 
if  the  soil  is  warm  enough  to  induce  reasonably  quick 
nitrification,  and  when  large  quantities  of  nitrate  are 
wanted  they  should  be  put  on  by  successive  applications 
of  not  more  than  i  cwt.  per  acre  at  a  time. 


CHAPTER   IX 

CAUSES  OF   FERTILITY   AND  STERILITY   OF   SOILS 

Meaning  of  Fertility  and  Condition — Causes  of  Sterility  :  Drought, 
Waterlogging,  Presence  of  Injurious  Salts — Alkali  Soils  and 
Irrigation  Water— Effect  of  Fertilisers  upon  the  Texture  of 
the  Soil — The  Amelioration  of  Soils  by  Liming,  Marling, 
Claying,  Paring,  and  Burning — The  Reclamation  of  Peat 
Bogs. 

IN  discussing  the  question  of  fertility,  a  difficulty  at  the 
outset  crops  up  in  the  definition  of  the  term  "  fertility  "  : 
we  are  dealing  with  something  intangible  and  dependent 
upon  so  many  varying  factors  that  it  becomes  a  matter 
of  judgment  and  experience  rather  than  of  scientific 
measurement.  We  have  to  distinguish  between  the 
fertility  proper,  "the  inherent  capabilities  of  the  soil," 
to  use  the  language  of  the  old  Agricultural  Holdings 
Act,  which  is  the  property  of  the  landlord,  and  for 
which  the  tenant  pays  rent ;  and  the  "  condition "  or 
"  cumulative  fertility,"  the  more  temporary  value  which 
is  made  or  marred  by  the  tenant.  Though  in  the  main 
it  is  easy  to  feel  the  distinction,  it  is  often  difficult,  if 
not  impossible,  to  draw  a  line  of  demarcation  between 
them.  Clearly  the  farmer  in  a  new  country  on  virgin 
soil  is  dependent  wholly  on  the  inherent  fertility  of 
the  land,  but  with  much  of  the  land  in  this  country  it 
is  hard  to  say  how  far  its  value  is  inherent,  or  due  to 
long-continued  cultivation.  When  a  tenant  by  in  any 


234    CAUSES  OF  FERTILITY  AND  STERILITY  [CHAP. 

years  of  skilful  management  makes  a  good  pasture, 
the  improvement  is  rightly  credited  to  him,  as  fertility 
which  he  has  accumulated :  the  next  tenant  must 
regard  the  same  pasture  as  part  of  the  inherent  capacity 
of  the  soil.  Again,  a  farmer  working  on  the  old  four 
course  rotation,  selling  only  corn  and  meat  and  purchas- 
ing neither  feeding  stuffs  nor  manures,  is  dependent  on 
the  fertility  of  the  soil :  another  farmer,  carrying  on  an 
agricultural  business  such  as  market  gardening  or  hop- 
growing,  and  putting  more  into  the  land  every  year  as 
manure  than  he  takes  out  as  crop,  is  only  using  the 
land  as  he  would  a  building,  as  a  tool  in  a  manufactur- 
ing process. 

Fertility  proper  is  by  no  means  a  wholly  chemical 
question,  dependent  upon  the  amount  of  plant  food  the 
soil  contains ;  in  many  cases  the  physical  conditions 
which  regulate  the  supply  of  air  and  water  to  the  plant, 
and  as  a  corollary,  the  bacterial  life,  are  far  more  potent 
in  producing  a  fertile  soil  than  the  mere  amount  of 
nutrient  material  it  contains.  Especially  is  this  the 
case  in  an  old  settled  country  like  England,  where 
manure  is  cheap  and  abundant ;  here  a  fertile  soil  is 
often  one  which  is  not  rich  in  itself,  but  one  that  is 
responsive  to,  and  makes  the  most  of,  the  manure 
applied.  Clay  soils  are  not  uncommon  which  show  on 
analysis  high  proportions  of  nitrogen  compounds  and 
potash,  and  again  no  particular  deficiency  in  phosphoric 
acid,  but  from  their  closeness  of  texture  they  offer  such 
resistance  to  the  movements  of  both  air  and  water  as  to 
carry  very  poor  crops.  Some  light  soils  again,  such  as 
those  on  the  chalk,  would  be  regarded  on  analysis  as 
rich,  but  they  are  made  so  persistently  dry  by  the  natural 
drainage,  that  only  in  a  wet  season  do  they  keep  the 
crop  sufficiently  supplied  with  water  for  a  large  crop 
production.  On  nearly  all  poor  soils  it  is  impossible  to 


IX.]  FERTILITY  235 

effect  much  improvement  by  the  use  of  manures ;  in  fact, 
manuring  will  not  turn  bad  into  good  land,  the  con- 
ditions limiting  the  amount  of  crop  being  other  than 
the  food  supply.  Of  course,  by  the  continued  incorpora- 
tion of  humus  into  a  light  soil,  its  physical  texture  may 
be  improved  at  the  same  time  as  its  richness,  until  it 
becomes  sufficiently  retentive  of  water  for  the  needs  of 
an  ordinary  crop,  just  as  a  heavy  soil  may  be  lightened 
by  similar  additions  of  humus.  It  has  already  been 
mentioned  that  many  subsoils,  especially  of  the  heavier 
loams  and  clays,  are  extremely  infertile  when  brought 
to  the  surface,  even  though  they  may  possess  a  fair 
proportion  of  phosphoric  acid  and  potash  and  be  arti- 
ficially supplied  with  nitric  nitrogen.  Some  of  this 
effect  is  due  to  texture,  part  to  the  very  scanty 
bacterial  flora  they  possess,  but  it  is  to  be  noted  that 
in  arid  climates  the  subsoils,  which  are  not  more  fine- 
grained than  the  surface  soils,  do  not  show  the  same 
infertility  when  brought  to  the  surface. 

The  soils  which  show  the  greatest  fertility  are,  as 
a  rule,  soils  of  transport,  uniform  and  fine-grained  in 
texture,  but  with  particles  of  a  coarser  order  than  clay 
predominating,  so  that,  while  lifting  water  easily  by 
capillarity,  they  are  freely  traversed  by  air  and  per- 
colating water.  As  a  rule,  they  also  contain  an  appreci- 
able amount  of  organic  matter  at  all  depths  ;  in  Britain 
they  have  been  deposited  from  running  water,  and 
represent  the  silt  from  which  both  the  coarsest  sand 
and  the  finest  clay  particles  have  been  sifted,  together 
with  a  certain  amount  of  vegetable  debris.  We  have 
nothing  comparable  with  the  typical  "black  soils"  of 
the  North  American  prairies  or  the  Russian  steppes, 
which  contain  very  large  proportions  of  organic  matter 
to  considerable  depths  in  the  subsoil :  as,  for  example, 
a  soil  from  Winnipeg  that  contained  0-428,  0-327,  0-158, 


236   CAUSES  OF  FERTILITY  AND  STERILITY  [CHAP. 

and  0107  per  cent,  of  nitrogen  in  the  top  4  feet  of 
soil  successively.  Many  of  these  deep  rich  soils  appear 
to  be  wind-borne  :  in  all  cases  they  are  of  very  uniform 
texture,  and  represent  the  accumulated  residues  of  ages 
of  previous  vegetation  in  a  form  that  is  capable  of 
decay  and  nitrification  so  as  to  become  available  for 
subsequent  crops.  In  a  peat  bog  there  is  equally  an 
accumulation  of  organic  matter  and  nitrogen,  but  the 
mass  is  infertile  because  of  the  acid  character  of  the 
humus  (which  causes  the  absence  of  the  valuable  bacteria, 
such  as  those  fixing  nitrogen  and  nitrifying  ammonia), 
the  deficiency  of  mineral  plant  foods,  and  the  bad 
mechanical  condition  which  affects  the  supply  of  air 
and  water.  In  the  main,  then,  a  fertile  soil  is  one  rich  in 
the  debris  of  previous  vegetation,  one  which  has  been  so 
sorted  out  by  running  water,  wind,  the  agency  of  worms, 
etc.,  as  to  possess  a  very  uniform  texture,  adapted  to 
satisfy  the  needs  of  the  plant  for  air  and  water. 
Mechanical  texture  is  of  fundamental  importance :  in 
this  country  many  soils  owe  their  value  to  this  property 
alone ;  for  example,  the  Thanet  Sand  formation  in  East 
Kent  (a  very  fine-grained  sand  or  silt),  though  it  con- 
tains but  little  plant  food,  yet  carries  some  of  the  best 
fruit  and  hop  plantations  in  the  kingdom,  and  farms  on 
it  command  a  high  rent 

Condition. 

The  question  of  condition  has  equally  its  chemical 
and  its  mechanical  side ;  it  is  well  known  that  on 
any  but  the  lightest  soils  continued  cultivation  makes 
the  texture  better  and  renders  it  easier  to  obtain  a 
seed  bed.  On  clay  soils  the  effects  of  bad  manage- 
ment are  very  persistent ;  any  ploughing,  rolling,  or 
trampling  when  the  soil  is  wet  will  so  temper  the 


IX.]  CONDITION  237 

clay  that  the  effect  is  palpable  until  the  land  has  been 
fallowed  again  or  even  laid  down  to  grass.  Once 
protected  from  the  action  of  frost,  stiff  soil  which  has 
been  worked  when  at  all  wet  never  seems  able  to 
recover  its  texture,  as  may  be  seen  by  examining  the 
clods  that  are  to  be  found  on  digging  up  an  old  post, 
the  result  of  the  trampling  when  the  post  was  originally 
put  in.  The  dependence  of  "  condition "  upon  the 
maintenance  of  a  good  texture  is  to  be  seen  in  the 
custom  of  regarding  wheat  as  an  exhausting  crop, 
whereas  few  of  our  farm  crops  withdraw  less  plant 
food  from  the  soil.  The  popular  opinion  really 
represents  the  fact  that  the  wheat  crop  occupies  the 
land  for  nearly  a  year  during  which  period  it  receives 
little  or  no  cultivation  and  so  falls  into  a  poor  state  of 
tilth. 

From  the  chemical  side  "condition"  means  the 
accumulation  within  the  soil  of  compounds  that  will 
by  normal  decay  yield  sufficient  available  plant  food 
for  the  requirements  of  an  ordinary  crop,  e.g.,  of 
organic  compounds  of  nitrogen  which  readily  nitrify, 
of  phosphoric  acid  and  potash  compounds  which  readily 
become  "  available  "  for  the  plant. 

The  condition  of  land  cannot  be  restored  all  at  once 
by  manuring ;  the  residues  of  manures  left  in  the  soil 
after  the  first  season  are  slow-acting,  i.e.,  only  a  small 
proportion  of  them  becomes  available  year  by  year, 
so  that  there  must  be  a  considerable  accumulation  of 
such  residues  before  the  proportion  becoming  avail- 
able during  the  period  of  growth  is  sufficient  for  the 
crop.  Per  contra,  the  condition  can  be  only  too  easily 
destroyed  by  cropping  without  manure ;  the  unexhausted 
residue  left  after  each  year  is  successively  less  and  less 
active,  the  crop  falls  off  rapidly,  till  at  last  a  sort  of 
stationary  condition  is  reached,  and  the  somewhat  inert 


238    CAUSES  OF  FERTILITY  AND  STERILITY  [CHAP. 

materials,  still  left  in  large  quantity,  liberate  year  by  year 
a  fairly  constant  proportion  of  active  plant  food.  The 
plots  at  Rothamsted  which  have  been  cropped  without 
manure  for  more  than  fifty  years  show  but  little  less 
average  production  during  the  last  twenty  years  than  in 
the  twenty  immediately  preceding.  For  example,  the 
unmanured  wheat  plot  shows  the  following  crop  in 
bushels  of  dressed  grain  : — 


First  28  years, 
1852-74. 

Second  28  years, 
1875-97. 

1898. 

1899. 

1900. 

144 

"I 

"» 

12 

12* 

Condition  may  best  be  regarded  as  a  state  of  equi- 
librium when  the  land  will  continue  to  give  a  good  return 
in  crop  for  the  manure  applied ;  as  a  rule,  the  crop 
recovered  by  no  means  contains  the  whole  of  the 
material  applied  as  manure,  a  certain  portion  being 
retained  in  a  comparatively  inactive  form.  With  the 
land  in  condition  the  remaining  nutrient  material 
required  for  a  good  crop  is  supplied  by  the  dormant 
residues  in  the  soil  which  have  become  active  :  at  the 
same  time,  these  reserves  are  protected  from  depletion 
by  renewal  from  the  inactive  portions  of  the  current 
manuring.  On  the  other  hand,  if  the  land  is  in  poor 
condition  the  crop  gets  little  or  no  assistance  from 
the  soil,  but  is  grown  from  the  active  part  only 
of  the  manure :  the  rest  of  it  accumulates  and  begins 
to  build  up  condition,  which,  however,  does  not  tell 
on  the  yield  for  some  time.  As  a  practical  conse- 
quence, it  is  noticed  that  only  land  in  good  condition 
gives  a  paying  return  year  after  year  for  the  application 
of  manure :  yet  if  the  experiment  be  made  of  omitting 
the  manure  on  a  portion  of  the  land  for  one  year, 
there  is  little  corresponding  reduction  of  yield,  as 


IX.]  FAIR  V  RINGS  239 

though  the  manure  went  to  keep  up  the  "condition," 
and  the  crop  was  grown  out  of  that  rather  than  from 
the  manure  applied. 

From  the  point  of  view  of  analysis  the  estimation  of 
the  "  condition  "  of  a  given  piece  of  land  is  a  difficult 
matter  on  which  light  is  only  just  beginning  to  be 
thrown  by  the  determination  of  "  available  "  plant  food, 
such  as  the  nitrates  and  the  phosphoric  acid  and  potash 
soluble  in  dilute  acid  solvents.  By  considering  such 
factors  as  these  and  the  amount  of  humus  soluble  in 
alkali,  the  ratio  of  the  soil  carbon  to  the  nitrogen, 
and  the  proportion  of  calcium  carbonate,  the  agricul- 
tural chemist  may  form  an  idea  as  to  the  immediate 
state  of  the  land.  Doubtless,  the  prevalence  and  dis- 
tribution of  such  necessary  bacteria  as  those  causing 
nitrification  are  also  important  factors  in  determining 
the  fertility,  but  on  this  point  we  are  without  exact 
information.  It  will  be  seen  that  "condition"  is  one 
of  the  most  valuable  of  the  properties  of  the  soil  to 
the  cultivator ;  as  it  may  be  destroyed  or  created  by 
the  tenant  during  his  occupation  of  the  land,  it  should 
be  as  far  as  possible  a  tenant's  asset,  to  be  bought  by 
him  on  entry  and  valued  to  him  on  leaving.  The 
difficulty  which  even  an  experienced  man  finds  in 
putting  a  value  on  so  intangible  an  item  makes  it 
almost  impossible  to  assess  the  condition  of  a  farm, 
but  it  is  desirable  in  every  way  that  the  outgoing 
tenant  should  be  encouraged  to  maintain  the  condition 
of  his  farm  by  giving  him  due  compensation  for  the 
unexhausted  value  both  of  manures  and  foods  used 
in  the  latter  years  of  his  tenancy. 

Fairy  Rings. 

The  significance  of  "condition"  and  its  dependence 
upon  a  supply  of  recently  decayed  organic  matter  is 


240    CAUSES  OF  FERTILITY  AND  STERILITY  [CHAP. 

well  seen  in  the  development  of  "  fairy  rings "  in 
pastures.  "  Fairy  rings "  are  circles  of  dark  -  green 
grass,  common  enough  in  poor  pastures,  which  are 
found  to  extend  their  size  every  year,  leaving  the  grass 
within  the  ring  of  a  lighter  colour  and  of  generally 
poorer  aspect  than  that  outside.  On  examining  the 
soil  immediately  outside  a  ring,  it  is  found  to  be  full  of 
the  mycelium  of  one  or  two  common  species  of  fungi, 
but  the  mycelium  rarely  occurs  in  the  soil  beneath  the 
ring  itself,  and  never  in  that  within  the  ring.  The 
ring  appears  to  be  dependent  on  the  growth  of 
the  fungus,  which  starts  at  one  point  and  draws 
upon  the  humus  reserves  contained  in  the  soil. 
Having  consumed  whatever  humus  is  available,  the 
mycelium  must  proceed  into  the  annular  area  of  soil 
immediately  round  the  first  patch,  thus  from  year  to 
year  it  spreads  outward.  After  the  death  of  the 
fungus,  there  is  left  behind  in  the  soil  it  has  just 
occupied  a  quantity  of  organic  matter,  which  readily 
decays  and  becomes  available  for  plant  nutrition ; 
thus  a  ring  of  luxuriant  vegetation  immediately 
follows  the  death  of  the  fungus.  In  other  words, 
the  humus  of  the  soil,  slow  to  decay  and  nitrify  in 
the  usual  way,  is  changed  into  material  undergoing 
rapid  change  by  its  preliminary  conversion  into  the 
tissue  of  the  fungus.  At  the  same  time,  as  the 
supply  of  rapidly  acting  plant  food  has  been  solely 
derived  from  the  soil,  the  ultimate  result  is  the  im- 
poverishment of  the  soil  within  the  ring  by  the  develop- 
ment of  the  fungi  and  the  subsequent  luxuriant  growth 
of  grass. 

The  following  figures  relate  to  the  composition  of 
the  soil  (mean  of  five  examples)  within,  on,  and  outside 
fairy  rings : — 


IX.] 


CAUSES  OF  STERILITY 


241 


Carbon  per 
cent. 

Nitrogen  per 
cent. 

Nitrates  per 
million. 

Outside  the  ring 
On  the  ring 
Inside  the  ring. 

3-30 

2-99 
2-78 

0-281 
0-266 
0-247 

2-44 

11-46 
103 

It  will  be  seen  that  the  unchanged  soil  outside  con- 
tains the  most  carbon  and  nitrogen ;  the  ring  itself 
contains  an  intermediate  amount,  and  the  least  is 
contained  within  the  ring  after  the  luxuriant  vegeta- 
tion has  passed  away.  The  soil  on  the  ring  is  in  high 
condition,  because  the  organic  residues  it  contains  are 
recently  formed  and  will  change  rapidly ;  after  they 
have  been  cropped  out,  the  land  is  less  able  to  support 
a  crop,  even  though  there  is  still  much  plant  food  left 
in  the  soil.  The  last  column  in  the  table  (the  analysis 
of  a  single  example  only)  shows  the  difference  in  avail- 
able nitrogen  ;  and  though  in  a  pasture  there  are  never 
many  nitrates  to  be  detected,  so  rapidly  are  they  seized 
upon  by  the  crop,  still  the  organic  nitrogen  compounds 
in  the  soil  must  be  in  a  more  nitrifiable  condition  on 
the  ring  to  yield  the  results  there  shown.  Doubtless  an 
investigation  of  the  nature  and  distribution  of  the 
bacteria  and  micro-fungi  in  and  about  a  fairy  ring  would 
throw  further  light  on  the  varying  fertility  of  such  closely 
neighbouring  areas  of  soil,  but  no  data  are  at  present 
available. 

Sterility  of  Soils. 

Few  soils  occurring  in  this  country  can  be  described 
as  absolutely  barren,  yet  from  time  to  time  land  is  met 
with  which  yields  such  poor  crops  that  it  may  fairly  be 
designated  as  sterile.  The  causes  of  sterility  are 
various ;  amongst  them  may  be  enumerated  both  the 

Q 


242    CAUSES  OF  FERTILITY  AND  STERILITY  [CHAP. 

want  and  the  excess  of  water  due  to  texture  and 
situation,  deficient  aeration,  the  absence  of  calcium 
carbonate,  and  the  toxic  action  of  certain  com- 
pounds, such  as  the  salts  of  magnesia,  iron  pyrites  or 
ferrous  salts  generally,  and  common  salt  itself.  An 
acid  reaction  of  the  soil,  which  is  highly  prejudicial  to 
vegetation,  is  generally  brought  about  by  one  or  other  of 
the  causes  enumerated  above. 

The  sterility  brought  about  by  a  deficiency  of  water 
is  only  seen  in  this  country  when  the  soil  is  so  entirely 
composed  of  coarse  sand  that  it  possesses  no  retentive 
power  for  the  rainfall ;  even  then  the  absolutely  bare 
condition  does  not  persist  long,  and  may  be  attributed 
as  much  to  the  lack  of  nutriment  as  to  the  want  of 
water.  Little  by  little  vegetation  is  found  to  creep  over 
recent  deposits  of  coarse  sea-sand  and  shingle,  until  a 
turf  is  established.  As  a  rule,  such  deposits  have  perma- 
nent water  at  a  comparatively  short  distance  below 
and  by  this  the  vegetation  is  maintained ;  but  where  a 
coarse,  open-textured  sand  occupies  the  uplands,  as  on 
the  Bagshot  and  Lower  Greensand  formations  of  the 
south  of  England,  or  the  Bunter  beds  of  the  Midlands, 
the  soil  is  kept  so  poor  that  it  has  largely  remained 
common  heath  land,  never  having  been  worth  the 
expense  of  enclosing.  Allusion  has  already  been  made, 
under  the  head  of  drainage,  to  the  evils  which  ensue  in 
a  waterlogged  soil :  from  time  to  time  clays  are  met 
with  of  so  close  a  texture  that  the  vegetation  suffers  in 
an  analogous  manner  through  deficient  aeration.  On 
certain  areas  of  the  Oxford  Clay  and  London  Clay,  and 
the  Boulder  clays  derived  therefrom,  pastures  degenerate 
after  a  few  years  into  a  mass  of  creeping  rooted  plants 
like  bent  grass,  and  the  land  must  be  broken  up  afresh 
in  order  to  aerate  it  before  any  crop  can  be  grown. 

Sterility  due   to  chemical   causes   is  perhaps  most 


IX.]  CAUSES  OF  STERILITY  243 

generally  caused  in  this  country  by  the  absence  of 
calcium  carbonate  from  the  soil.  When  this  happens 
on  light  sandy  land  it  will  become  evident  by  the 
tendency  of  black  mild  humus  to  accumulate,  by  the 
paucity  of  leguminous  plants  in  the  herbage,  and  by  the 
prevalence  of  fungoid  diseases  like  "  finger-and-toe." 
On  strong  lands,  and  when  accompanied  by  water- 
logging, black  acid  peat  accumulates :  the  soil  shows 
an  acid  reaction,  oxide  of  iron  forms  below  the  surface, 
and  the  soil  water  contains  soluble  iron  salts,  as  is  seen 
by  the  iridescent  scum  which  spreads  over  any  water 
standing  in  the  ditches. 

Another  source  of  sterility  is  the  presence  of  un- 
oxidised  iron  salts  in  the  soil :  many  clay  subsoils  are 
coloured  dark  blue  or  green  by  double  ferrous  silicates 
like  glauconite,  or  by  finely  disseminated  iron  pyrites. 
Until  these  materials  become  oxidised  to  ferric  hydrate, 
the  soil  remains  sterile :  particularly  is  this  the  case 
with  iron  pyrites,  which  in  the  form  of  marcasite  easily 
oxidises  to  yield  both  ferrous  sulphate  and  sulphuric 
acid.  Voelcker  has  recorded  three  instances  of  soil 
sterile  through  these  causes :  one  was  land  reclaimed 
from  the  bed  of  the  Haarlem  Lake,  which  contained  0-71 
per  cent,  of  iron  pyrites  and  0-74  per  cent,  of  ferrous 
sulphate,  as  well  as  some  insoluble  basic  sulphate  of 
iron.  Another  example  of  land  reclaimed  from  the 
sea  contained  0-78  per  cent,  of  pyrites  and  1-39  per 
cent,  of  ferrous  sulphate.  Cultivation  with  a  free  use 
of  lime  and  chalk  is  the  best  means  of  ameliorating 
such  soils,  which  always  show  an  acid  reaction. 

Kearney  and  Cameron  in  America  have  shown  that 
salts  of  magnesia  possess,  even  in  solutions  of  great 
dilution,  a  toxic  action  upon  plant  roots,  which  is  much 
diminished  if  calcium  salts  be  present  at  the  same  time. 
Loew  at  the  same  time  has  indicated  that  a  comparative 


244     CAUSES  OF  FERTILITY  AND  STERILITY  [CHAP. 

excess  of  magnesium  over  calcium  in  certain  soils 
results  in  sterility.  With  this  may  be  correlated  the 
fact  that  the  soils  resting  upon  the  serpentine,  which 
is  a  compound  containing  magnesia,  are  notoriously 
poor,  also  that  certain  very  impoverished  clays  on  the 
Wealden  formation  contain  a  high  proportion  of 
magnesia. 

Sterility  caused  by  salt  is  sometimes  to  be  seen  in  this 
country  in  the  marshes  near  the  sea :  more  often  a  "  salt- 
ing," even  where  the  sea  has  regular  access,  is  clothed 
with  vegetation  which  is  able  to  endure  very  consider- 
able proportions  of  salt.  Most  farm  crops  will  grow  in 
soil  containing  0-25  per  cent,  of  salt,  and  in  the  reclaiming 
of  the  old  sea  lake  of  Aboukir  in  Egypt,  it  was  found 
that  grasses  would  grow  freely  when  there  was  still  as 
much  as  I  per  cent,  of  salt  in  the  soil,  and  a  scrubby 
winter  crop  of  barley  was  grown  on  soil  containing  more 
than  i  £  per  cent.  "  With  2  per  cent,  of  salt  in  the  soil,  a 
fair  crop  of  dineba  (grass),  2  feet  high,  can  be  grown  ; 
with  i  per  cent,  it  attains  its  full  height  of  4  feet,  and 
sells  as  a  standing  crop  at  from  2Os.  to  255.  per  acre. 
For  '  berseem,'  or  clover,  the  percentage  of  salt  should 
not  exceed  |,  and  about  the  same  for  '  sabaini  (quick- 
growing  or  seventy-day)  rice.' "  Much,  however,  depends 
upon  the  relations  between  water  supply  and  evapora- 
tion, as  to  the  amount  of  salt  in  a  soil  which  would  be 
tolerable  to  vegetation.  From  time  to  time  cases  occur 
in  this  country  of  crops  being  destroyed,  and  land 
rendered  sterile  by  the  incursion  of  sea  water ;  the  effect 
is  not  always  apparent  at  first,  though  sea  water  contains 
as  much  as  2-7  per  cent,  of  sodium  chloride  and  0-5  per 
cent,  of  other  soluble  salts,  but  the  permanent  pasture 
becomes  seriously  injured,  and  for  two  or  three  years 
even  the  arable  land  yields  very  indifferent  crops. 
Dymond  has  attributed  this  after  effect  to  the  injurious 


IX.]  ALKALI  SOILS  245 

action  of  the  sea  water  on  the  texture  of  the  soil,  due  to 
the  attack  of  the  sodium  chloride  upon  the  double 
silicates  of  the  soil,  lime  in  particular  being  displaced 
by  soda.  The  result  is  the  deflocculation  of  the  clay, 
which  will  not  settle  down  for  many  weeks  when  sus- 
pended in  water.  The  sodium  chloride  of  the  sea  water 
would  also  interact  with  any  calcium  carbonate  in  the 
soil,  giving  rise  to  sodium  carbonate,  the  deflocculating 
effect  of  which  upon  the  clay  has  already  been  noticed. 
Biological  effects  may  also  be  surmised :  it  is  always 
seen  that  the  earth  worms  are  killed  in  the  land  which 
has  been  flooded  with  sea  water,  and  in  view  of  the 
known  unfavourable  effect  of  chlorides  on  nitrification, 
it  is  possible  that  the  rate  of  production  of  nitrates  in 
the  inundated  soil  is  seriously  lessened. 

Alkali  Soils. 

In  arid  climates  the  rainfall  is  often  insufficient  to 
produce  percolation  through  the  soil  and  subsoil  into  the 
underground  water  system  ;  in  consequence,  the  salts 
produced  by  the  weathering  of  the  rocks  tend  to 
accumulate  in  the  subsoil,  and  may  be  brought  to  the 
surface  by  capillary  rise  so  as  to  cause  almost  entire 
sterility.  Such  bad  lands  are  known  in  America  as 
"  alkali  soils,"  but  they  are  well  known  in  India  and  in 
Egypt,  and  indeed  are  common  to  all  countries  possess- 
ing a  small  rainfall  and  great  evaporation.  In  its  most 
aggravated  form  alkali  land,  particularly  at  the  end  of 
the  dry  season,  shows  an  actual  white  efflorescence  of 
salts  at  the  surface  ;  all  vegetation  is  destroyed,  except 
one  or  two  plants  which  seem  tolerant  of  large  quantities 
of  saline  matter,  such  as  "  greasewood,"  Sarcobatus  sp., 
or  the  Australian  "  saltbushes,"  Atriplc.v  semibaccatum, 
etc.  In  some  cases  the  alkali  is  chiefly  located  at  a 
slight  depth  in  the  soil,  and  only  effloresces  on  spots  a 


246     CAUSES  OF  FERTILITY  AND  S 


little  below  the  generatigvel,  where  the  sugffi  water 
comes  to  the  surface.  A  heavy  rainfall  may  be  followed 
by  a  rise  of  alkali,  because  a  connection  is  then 
established  between  the  saline  subsoil  water  and  the 
evaporating  surface,  whereupon  a  continuous  capillary 
use  of  salts  takes  place,  followed  by  their  crystallisation 
at  the  surface.  Per  contra,  the  establishment  of  a  soil 
mulch,  and  shading  the  ground  with  a  crop,  so  that 
evaporation  only  takes  place  through  the  leaves,  will  aid 
in  keeping  the  alkali  down.  The  composition  of  the 
salts  varies  ;  as  a  rule,  sodium  chloride  predominates, 
with  some  sulphates  of  sodium,  magnesium,  and  calcium, 
in  which  case  the  material  is  known  as  "  white  alkali." 
Under  other  conditions  the  material  is  really  alkaline, 
containing  carbonate  and  bicarbonate  of  soda  ;  the 
saline  solution  then  dissolves  some  of  the  humus  present 
in  the  soil,  and  also  causes  the  resolution  of  the  clay 
material  into  its  finest  particles,  so  that  the  soil  forms 
an  intensely  hard  black  pan  when  dry,  which  is  known 
as  "black  alkali."  The  carbonates  are  far  more 
injurious  to  vegetation  than  the  neutral  salts  ;  few 
plants  can  bear  as  much  as  01  per  cent,  of  sodium 
carbonate,  but  are  tolerant  of  0-5  to  I  per  cent,  of 
the  other  salts. 

Though  the  alkali  salts  are  sometimes  chiefly 
sulphates,  more  commonly  sodium  chloride  is  the 
main  constituent,  together  with  the  products  of  its 
action  in  mass  upon  calcium  carbonate  and  sulphate. 
The  diagram  (Fig.  16),  due  to  Hilgard,  shows  the  dis- 
tribution with  depth  of  alkali  salts  in  this  type  of  soil 
at  Tulare,  California  ;  the  greatest  accumulation  of  salts 
takes  place  at  a  depth  of  30  inches,  the  point  to  which 
the  annual  rainfall  penetrates.  One  of  the  most  difficult 
features  presented  by  the  cultivation  of  land  in  arid 
regions  where  alkali  occurs  in  the  soil,  comes  from  the 


FlO.    j6. — Nature  and  Distribution  of  Alkali  Salts  (Hilgard). 

[To  fact  page  240. 


ix.]  EViyrDUE  TO  IRRIGATION  247 

tendency  of  U>€  sterile  spots  to  spread  and  the  alkali  to 
be  broughf'to  the  surface  as  soon  as  irrigation  water  is 
cmplpwffi:  for  without  irrigation  agriculture  is  nardiv 
possHSle.  Many  districts,  which  at  first  carried  good 
crops  and  were  even  laid  down  in  fruit  or  vines,  have  been 
ruined  through  the  rise  of  alkali  to  the  surface  brought 
about  by  irrigation ;  in  fact,  in  all  these  arid  regions  it 
becomes  exceedingly  dangerous  to  raise  the  water  table 
in  the  land  anywhere  near  the  surface,  because  capillarity 
then  causes  a  rise  of  the  salt  changed  water,  and  evapora- 
tion concentrates  it  on  the  top.  Just  as  some  of  the  worst 
alkali  land  occurs  where  rain  falling  upon  the  surround- 
ing mountains  finds  its  way  by  seepage  through  the 
subsoil  rich  in  salts  and  then  rises  to  the  surface  in  the 
dry  basin  areas  below,  so  the  introduction  of  irrigation 
canals  pouring  large  volumes  of  water  upon  the  land, 
may  equally  establish  the  capillary  connection  between 
the  subsoil  salts  and  the  surface.  The  following  extract 
from  Bulletin  No.  14,  U.S.  Dept.  of  Agric.,  Div.  of  Soils, 
dealing  with  alkali  soils  in  the  Yellowstone  Valley, 
shows  the  evil  effects  of  incautious  irrigation  : — 

"  Irrigation  has  been  practised  for  twelve  or  fifteen 
years.  The  water  for  the  main  ditch  supplying  the 
valley  is  taken  out  of  the  river  nearly  40  miles  above 
the  town  of  Billings.  When  the  country  was  first  settled, 
and  indeed  above  the  ditch  at  the  present  time,  the 
depth  to  standing  water  in  the  wells  was  from  20  to  50 
feet,  and  there  was  no  signs  of  alkali  on  the  surface  of 
the  ground.  Under  the  common  practice  of  irrigation, 
however,  an  excessive  amount  of  water  has  been  applied 
to  the  land,  and  seepage  waters  have  accumulated  to 
such  a  degree  that  water  is  now  secured  in  wells  at  a 
depth  of  from  3  to  10  feet  in  the  irrigated  district,  while 
many  once  fertile  tracts  on  the  lower  levels  are  already 
flooded,  and  alkali  has  accumulated  on  them  to  such  an 


248     CAUSES  OF  FERTILITY  AND  STERILITY  [CHAP. 

extent  that  they  are  mere  bogs  and  swamps  and  alkali 
flats,  and  the  once  fertile  lands  are  thrown  out  as  ruined 
and  abandoned  tracts." 

Nor  is  it  necessary  that  the  subsoil  be  charged  with 
salts  for  irrigation  to  produce  alkali  land ;  the  mere 
continual  evaporation  of  ordinary  river  or  spring  water 
may  cause  such  an  accumulation  of  saline  matter  at  the 
surface  as  is  harmful  to  vegetation.  This  is  well  seen  in 
Egypt,  where  perennial  irrigation  is  practised  with  the 
Nile  water,  and  the  following  quotation  from  Willcock's 
Egyptian  Irrigation  will  explain  the  action  that  takes 
place : — 

"  The  introduction  of  perennial  irrigation  into  any 
tract  in  Egypt  means  a  total  change  in  crops,  irrigation, 
and  indeed  everything  which  affects  the  soil.  Owing  to 
the  absence  of  rain,  the  land  is  not  washed  as  it  is  in 
other  tropical  countries,  unless  it  is  put  under  basin 
irrigation. 

"  An  acre  of  land  may  receive  as  many  as  twenty 
waterings  of  about  9  cm.  in  depth  each,  i.e.,  a  depth 
of  water  of  1-80  metre  per  annum,  which  is  allowed 
to  stand  over  the  soil,  sink  about  half  a  metre  into 
the  soil,  and  then  be  evaporated.  Since  the  Nile 
water,  especially  in  summer,  has  salts  in  excess,  these 
salts  accumulate  at  the  surface,  and  if  not  eaten  down 
by  suitable  crops,  soon  appear  as  a  white  efflorescence. 
While  the  spring  level  is  low,  capillary  attraction 
cannot  bring  up  to  the  surface  the  spring  water,  which 
generally  contains  a  fair  proportion  of  salts,  but  where 
the  spring  level  is  high  the  salt-carrying  water  comes 
to  the  surface,  is  there  evaporated,  and  tends  to  further 
destroy  the  soil.  In  old  times  the  greater  part  of 
the  cultivation  land  was  under  basin  irrigation,  and 
was  thoroughly  washed  for  some  fifty  days  per  annum  ; 
while  the  rest,  consisting  of  the  light  sandy  soils  near  the 


ix.]      IRRIGATION  NECESSITATES  DRAINAGE      249 

Nile  banks,  was  protected  by  insignificant  dykes,  which 
dykes  were  burst  every  very  high  flood,  and  thus  allowed 
to  be  swept  over  by  the  Nile  and  washed  once  every 
seven  or  eight  years.  All  this  is  at  an  end  now  in 
the  tracts  under  perennial  cultivation,  and  other  remedies 
have  to  be  found." 

The  only  remedy  for  the  evils  attending  irrigation 
is  the  introduction  of  drainage  channels  at  a  lower 
level  than  the  canals  bearing  the  irrigation  water ;  in 
this  way  the  percolation  through  the  soil,  which  in 
humid  climates  naturally  removes  the  salts  not  taken 
up  by  the  crops,  is  effected  artificially ;  there  is  some 
apparent  loss  of  water,  but  this  is  absolutely  necessary 
to  maintain  the  land  free  from  injurious  salts.  As 
an  example,  the  following  passage  may  be  quoted 
from  one  of  Major  H anbury  Brown's  reports  on 
Egyptian  Irrigation : — 

"It  has  been  ascertained  that  the  blessing  of 
improved  water  supply  which  has  resulted  from  the 
barrage  having  been  made  to  do  its  duty,  has  been 
attended  in  some  localities  with  the  evils  due  to 
infiltration  and  want  of  drains.  The  remedy,  as  pointed 
out  in  last  year's  Report,  is  to  remove  the  want  of 
drains  by  digging  them,  and  to  provide  the  means  of 
washing  out  the  salt  brought  to  the  surface,  by  infil- 
tration in  the  shape  of  a  liberal  supply  of  water,  by 
which  the  salt  would  be  carried  away  in  solution  along 
the  drains,  or  be  forced  down  below  the  surface  of  the  soil 
to  a  depth  at  which  it  would  be  harmless.  The  liberal 
water  supply  is  not  to  be  obtained  except  by  the  con- 
struction of  a  storage  reservoir  at  Aswan  or  elsewhere." 

It  was  the  neglect  of  drainage,  when  irrigation 
canals  were  introduced,  that  led  to  so  widespread  a 
deterioration  of  land  in  Egypt.  To  quote  from  Lord 
Milner's  England  in  Egypt ; — 


250    CAUSES  OF  FERTILITY  AND  STERILITY  [CHAP. 

"  But  perhaps  the  worst  feature  of  all  was  the 
neglect  of  drainage,  which  was  steadily  ruining  large 
tracts  of  country.  Even  where  drains  existed,  they 
were  frequently  used  also  as  irrigation  channels,  than 
which  it  is  impossible  to  conceive  a  worse  sin  against 
a  sound  principle  of  agriculture.  In  some  cases  these 
channels  would  be  flowing  brimful  for  purposes  of 
irrigation,  just  when  they  should  have  been  empty 
to  receive  the  drainage  water.  Elsewhere  the  salt- 
impregnated  drainage  water  was  actually  pumped  back 
upon  the  land. 

"  It  was  the  want  of  drainage  which  completed  the 
ruin  of  the  Birriya,  that  broad  belt  of  land  which 
occupies  the  northern  and  lowest  portion  of  the  Delta, 
adjoining  the  great  lakes.  There  are  upwards  of 
1,000,000  acres  of  this  region,  now  swamp,  or  salt 
marsh,  or  otherwise  uncultivable,  which  in  ancient 
times  were  the  garden  of  Egypt." 

It  has  been  the  business  of  the  English  irrigation 
officers  since  the  occupation  to  restore  and  improve 
the  drainage  system,  and  to  begin  the  reclamation  of 
the  salted  areas  by  cutting  drainage  canals  and  passing 
enough  of  the  .abundant  winter  flood  water  through 
the  soil  to  wash  out  the  salts  into  these  drains. 

Hilgard  in  California  has  also  indicated  that  it  is 
impossible  to  wash  the  salts  from  the  soil,  even  by 
leaving  the  water  to  stand  upon  the  surface  for  some 
time,  unless  provision  is  made  to  remove  the  salted 
water  by  underdrainage.  In  the  case  of  black  alkali, 
however,  the  soil  has  become  too  impervious  to 
allow  water  to  percolate  at  all ;  the  first  remedial 
measure  is  to  incorporate  considerable  quantities  of 
gypsum  with  the  soil ;  this  will  interact  with  the 
sodium  carbonate,  producing  sodium  sulphate  and 
calcium  carbonate,  at  the  same  time  precipitating  the 


IX.]   FERTILISERS  DESTROYING  THE  TEXTURE  251 

humus  in  a  flocculent  form.  If  now  unclerdrainage  be 
brought  into  practice  the  soluble  salts  can  be  washed 
through,  and  a  very  fertile  soil  results,  owing  to  the 
presence  of  the  finely  divided  humus  and  calcium  car- 
bonate. Where  underdrainage  is  hardly  practicable 
because  of  the  expense,  irrigation  water  should  be 
used  in  as  limited  amounts  as  possible,  and  every 
care  should  be  taken  to  keep  the  surface  tilled  and 
under  crop,  so  as  to  minimise  evaporation  from  the 
bare  ground.  In  humid  countries  like  our  own, 
damage  due  to  the  accumulation  of  salts  are  rare ; 
the  author  has,  however,  seen  one  case  where  the 
vegetation  of  a  lawn  was  destroyed  during  a  hot  dry 
spell  of  weather  by  continuously  applying  water  in 
quantities  which  never  washed  down  into  the  subsoil, 
but  evaporated  every  day.  An  efflorescence  practically 
identical  with  white  alkali  is  sometimes  seen  on  green- 
house borders,  which  are  constantly  watered,  but  never  in 
sufficient  quantities  to  cause  percolation  ;  and  gardeners 
again  are  familiar  with  the  check  of  growth  which 
sometimes  occurs  in  the  case  of  plants  long  in  one 
pot  and  constantly  watered  with  hard  water.  The 
remedy  is  to  water  from  time  to  time  so  heavily  as 
to  cause  abundant  percolation  and  thus  wash  all  the 
salts  out. 

Closely  related  to  some  of  the  phenomena  presented 
by  alkali  soils  are  certain  secondary  effects  upon  the 
texture  of  the  soil  which  are  produced  by  the  action  of 
some  of  the  salts  used  as  artificial  manures.  A  good 
friable  texture  in  a  heavy  soil  depends  upon  the  clay 
particles  being  generally  flocculated  and  gathered 
together  into  little  aggregates,  which  give  the  soil  a 
coarser  grain  until  they  are  resolved  into  their  ultimate 
particles  by  incautious  working  when  the  clay  is  in  a  wet 
state.  It  has  already  been  shown  that  acids  and  most 


2$2     CAUSES  OF  FERTILITY  AND  STERILITY  [CHAP 

soluble  salts,  particularly  those  of  calcium,  possess  a 
strong  flocculating  power,  whereas  the  soluble  alkalis — 
the  carbonates  and  hydrates  of  sodium,  potassium,  and 
ammonium — are  active  deflocculators,  causing  the  clay 
particles  to  separate  into  their  most  fine-grained  condition. 

It  has  long  been  recognised  that  large  or  frequent 
dressings  of  nitrate  of  soda  had  an  injurious  action  upon 
the  tilth  of  the  soil,  causing  it  to  remain  very  wet,  and 
then  to  dry  into  hard,  unkind  clods.  Since  nitrate  of 
soda  is  very  hygroscopic,  the  wetness  induced  in  the 
land  was  attributed  to  the  absorption  of  moisture  from 
the  atmosphere  by  the  nitrate  of  soda,  but  when  it  is 
considered  what  a  very  small  proportion  the  water 
absorbed  by  as  much  as  5  cwt.  of  nitrate  of  soda  would 
bear  to  the  hundred  tons  which  is  the  approximate 
weight  of  an  acre  of  soil  an  inch  thick,  it  is  obvious  that 
the  difference  in  water  content  so  induced  would  not  be 
sensible. 

Clay  soils,  in  fact,  which  have  been  treated  with 
nitrate  of  soda,  do  not  show  any  excess  of  water ;  but 
they  are  very  much  deflocculated,  as  may  be  ascertained 
by  comparing  the  appearance  after  standing  of  a  jar  of 
distilled  water  rendered  turbid  by  shaking  up  in  it  a 
gram  of  the  soil,  with  a  second  jar  in  which  the  water 
has  been  shaken  with  a  gram  of  the  same  soil  in  its 
normal  condition.  But  nitrate  of  soda  itself  possesses 
flocculating  powers  even  when  concentrated,  hence  the 
observed  deflocculation  can  not  be  due  to  the  direct  action 
of  the  fertiliser  upon  the  clay.  However  it  has  been 
found  that  when  plants  feed  upon  a  nutrient  solution 
containing  nitrate  of  soda,  an  excess  of  the  nitric  acid 
is  withdrawn  by  the  plant,  and  part  of  the  soda  is  left 
in  the  medium  combined  with  the  carbon  dioxide 
secreted  by  the  plant.  The  existence  of  this  soluble 
alkali  after  the  growth  of  the  plant  can  be  verified  by 


ix.]  BAD  TILTH  DUE  TO  FERTILISERS  253 

experiments  with  water  cultures,  it  can  also  be  extracted 
from  the  soil  of  the  Rothamsted  plots  which  have  for 
many  years  been  manured  with  nitrate  of  soda.  Small 
as  the  amount  may  seem  to  be,  it  is  quite  sufficient  to 
account  for  the  deflocculation  of  the  clay  and  the 
defective  tilth  observed  on  heavy  land  after  nitrate  of 
soda  has  been  used. 

The  bad  repute  of  nitrate  of  soda  as  exhausting  or 
scourging  the  land,  is  less  due  to  any  sensible  diminution 
in  the  stock  of  plant  food  in  the  soil  that  follows  its  use, 
than  to  the  deflocculation  it  sometimes  induces,  and  the 
consequent  deterioration  of  the  texture  of  the  soil. 
As  a  remedy  lime  is  not  effective,  since  it  is  an  alkali 
itself;  instead  the  nitrate  of  soda  should  be  used  in 
conjunction  with  acid  flocculating  manures  like  super- 
phosphate, or  a  mixture  of  nitrate  of  soda  and  sulphate 
of  ammonia  should  be  used  as  a  nitrogenous  manure, 
because  the  two  manures  will  act  upon  the  soil  in 
opposite  ways,  the  nitrate  of  soda  as  an  alkali  and  the 
sulphate  of  ammonia  as  an  acid.  Dressings  of  soot  are 
also  effective ;  not  only  does  it  assist  the  soil  mechanically, 
but  also  the  small  percentage  of  sulphate  of  ammonia  it 
contains  possesses  some  power  of  flocculating  the  clay. 

Other  fertilisers  which  give  rise  to  an  alkaline 
reaction  in  the  soil  are  sulphate  of  potash,  common 
salt,  and  other  soluble  salts  of  sodium  and  potassium, 
which  as  has  already  been  noticed  (p.  217)  interact 
with  calcium  carbonate  in  the  soil,  and  give  rise  to  a 
little  soluble  alkaline  carbonate.  The  injurious  effects 
of  sulphate  of  potash  upon  the  tilth  of  the  heavy  soil 
at  Rothamsted  is  very  evident  on  the  mangold  field, 
where  the  plots  receiving  this  fertiliser  every  year 
become  excessively  sticky  and  clinging  in  wet  weather, 
and  dry,  with  a  hard  caked  surface.  It  has  often  been 
noticed  that  applications  of  potash  salts  and  common 


254    CAUSES  OF  FERTILITY  AND  STERILITY  [CHAP. 

salt  have  depressed  instead  of  increasing  the  yield  ;  this 
may  probably  be  set  down  to  the  deterioration  of  tilth 
that  ensues  when  the  soil  is  heavy  and  also  contains 
calcium  carbonate.  Some  fertilisers,  on  the  contrary, 
aid  in  the  flocculation  of  clay  soils,  the  most  effective 
being  superphosphate,  which  is  acid  and  contains  gypsum, 
an  effective  flocculating  agent.  The  ammonium  salts, 
which  give  rise  to  free  acids,  in  consequence  of  the 
withdrawal  of  ammonia  by  the  moulds,  etc.,  living  in 
the  soil,  act  as  very  potent  flocculators,  and  at 
Rothamsted,  for  example,  give  rise  to  an  open  and 
friable  soil,  as  compared  with  the  neighbouring  plots 
receiving  nitrate  of  soda.  Lime,  which  is  the  chief 
flocculating  agent  employed  in  practice,  is  only  effective 
when  it  has  become  dissolved  as  bi-carbonate  in  the 
soil  water. 

Amendments  of  the  Soil. 

Many  soils,  without  being  absolutely  sterile,  carry 
very  poor  crops  until  their  physical  character  has  been 
altered  by  the  admixture  of  some  considerable  quantity 
of  one  or  other  of  the  constituents  of  a  normal  soil 
that  may  happen  to  be  originally  wanting.  These 
amendments  of  the  soil  by  the  mixture  of  other  soils 
date  from  the  time  that  enclosures  first  began  to  be 
made  ;  they  were  perhaps  at  their  height  during  the 
early  years  of  the  nineteenth  century,  after  the  middle 
of  which  they  rapidly  diminished  as  it  began  to  be 
less  and  less  remunerative  to  "  make "  land,  until  at 
the  present  time  the  fall  in  the  prices  of  produce  and 
the  rise  in  the  cost  of  labour  have  put  an  end  to  all 
such  operations.  Among  other  causes  of  this  neglect 
may  perhaps  be  set  down  the  increased  use  of  artificial 
manures ;  men  began  to  take  too  exclusively  a  chemical 
view  of  the  functions  of  the  soil,  and  shirked  expendi- 


IX.]  WARPING  355 

ture  which  did  not  seem  to  add  directly  any  food  for 
the  plant.  However,  it  is  probable  that  with  modern 
facilities  for  moving  earth  on  a  large  scale  by  steam 
power,  the  improvement  of  much  poor  land  might  even 
now  be  profitably  undertaken. 

The  operations  which  may  be  grouped  under  the 
head  of  "  amendments  of  the  soil,"  comprise — drainage, 
which  has  been  dealt  with  elsewhere  ;  the  marling  and 
claying  of  light  sands  ;  the  reclamation  of  peat  bogs ; 
the  improvement  of  clay  soils  by  liming  and  chalking, 
or  by  paring  and  burning ;  and  lastly,  the  creation  of 
new  alluvial  soils  by  warping. 

Warping. 

The  operation  of  "warping,"  or  "colmetage,"  is 
only  possible  in  the  vicinity  of  tidal  estuaries,  where 
lands  exist  below  the  level  of  high  water,  and  is  in 
this  country  practically  confined  to  the  estuaries  of 
the  Humber  and  Ouse.  Warping  is  carried  out  by 
the  construction  of  a  wide  drain  protected  by  sluices 
from  the  tidal  river  to  the  low  land,  which  is  first 
divided  by  embankments  into  compartments  of  various 
sizes  up  to  150  acres.  When  the  embankments  have 
become  consolidated,  the  flood  tide,  heavily  charged 
with  suspended  matter  which  is  really  fine  earth 
brought  down  by  the  river,  is  admitted  into  the 
compartment,  where  it  deposits  most  of  its  silt  and 
is  allowed  to  run  off  when  the  level  of  the  water  in 
the  river  has  fallen  during  the  ebb.  The  operation 
is  repeated  until  a  layer  of  silt  has  formed  i  to  3 
feet  thick  over  the  land,  which  is  then  dried  and 
brought  under  crop.  As  the  chief  deposit  is  always 
near  the  mouth  of  the  drain,  where  the  velocity  of 
the  silt-bearing  current  is  first  checked,  the  position 
of  the  inlet  must  be  shifted  about  to  secure  a 


256    CAUSES  OF  FERTILITY  AND  STERILITY   [CHAP. 

uniform  deposit  all  over  the  land  and  to  distribute 
the  valuable  fine  silt  which  settles  furthest  from 
the  inlet.  In  some  cases  the  sluice  gates  are  auto- 
matic, and  water  is  admitted  and  drawn  off  at 
every  tide,  but  in  others  only  every  other  tide  is 
admitted,  thus  giving  time  for  the  deposit  of  the  finer 
particles,  and  greatly  improving  the  character  of  the 
resulting  land.  As  a  rule,  only  the  spring  tides  are 
utilised,  because  the  suspended  matter  is  then  at  its 
maximum,  and  the  process  is  confined  to  the  summer 
months,  to  avoid  danger  from  flooding  when  there  is 
much  land  water  about.  In  exceptional  cases  land 
may  be  warped  2  or  3  feet  deep  in  one  year — from 
January  to  June — in  other  cases,  where  the  water  is 
less  charged  with  sediment,  or  the  land  is  at  a  higher 
level,  an  efficient  warping,  which  should  not  be  less 
than  1 8  inches  deep,  requires  three  or  four  years. 
When  finished,  the  land  is  allowed  to  dry  and  con- 
solidate, drainage  grips  are  then  thrown  out,  and  a 
light  crop  of  oats,  in  which  are  sown  clover  and  rye- 
grass,  is  taken ;  after  the  seeds  have  been  down  two 
years  the  land  is  generally  ready  to  carry  wheat. 
Warp  soils  are,  as  a  rule,  fertile,  and  noted  for 
growing  seed  corn  of  high  quality ;  they  are  to  all 
intents  and  purposes  artificial  alluvial  soils,  composed 
entirely  of  the  finer  sands  and  silts  without  much 
clay  material,  and  are  comparatively  rich  in  organic 
debris  and  other  plant  food,  except  perhaps  potash. 
The  fertilising  of  the  Egyptian  land  by  the  red  Nile 
flood  water,  the  formation  and  improvement  of  river 
meadows  by  winter  flooding,  are  both  analogous  to 
the  process  of  "  warping." 

Marling  and  Claying. 
Many  light  and  blowing  sands,  almost  too  pure  to 


IX.]  MARLING  257 

permit  of  any  vegetation,  have  in  their  immediate 
neighbourhood  a  bed  of  marl  or  clay  which  can  be 
easily  incorporated,  practically  creating  a  soil  where 
there  was  none  before.  Among  the  New  Red  Sand- 
stones of  Cheshire  and  the  Midland  Counties  beds  of 
true  marl  occur  and  were  at  one  time  enormously 
worked,  so  that  every  farm  and  almost  every  field  shows 
its  old  marl  pit ;  the  sandy  Lower  Greensand  soils  in 
the  Woburn  district  have  been  extensively  marled  from 
the  adjoining  Oxford  clay,  and  many  of  the  Norfolk 
soils  have  been  made  out  of  blowing  sands,  by  bringing 
up  the  clay  which  immediately  underlies  them.  The 
earlier  volumes  of  the  Journal  of  the  Royal  Agricultural 
Society  contain  numerous  accounts,  showing  how  much 
land  was  brought  into  cultivation  by  these  means  in 
the  first  half  of  the  nineteenth  century. 

"  He  that  marls  sand  may  buy  the  land, 
He  that  marls  moss  shall  suffer  no  loss, 
But  he  that  marls  clay  flings  all  away." 

The  usual  practice  in  Norfolk  was  to  open  pits  down 
to  the  marl  or  clay,  dig  and  spread  it  at  the  rate  of 
50  to  150  loads  to  the  acre  on  a  clover  ley  or  turnip 
fallow.  In  some  cases  trenches  were  opened  all  along 
the  field,  and  the  clay  thrown  out  on  either  side.  By 
the  action  of  the  weather,  drying  and  wetting,  followed 
by  frost,  the  clay  comes  into  a  condition  to  be  harrowed 
down,  after  which  it  can  be  ploughed  into  the  ground. 

The  effect  of  marling  or  claying  is  more  evident  after 
a  year  or  two  than  at  once,  because  the  fine  particles 
become  each  year  more  thoroughly  incorporated  with 
the  soil.  The  effects  are  to  be  seen  in  increased  crops,  the 
production  of  better  leys  and  pastures,  greater  resistance 
to  drought,  and  particularly  an  increased  stiffness  in  the 
straw  where  manures  are  used  to  grow  the  crop. 

K 


258    CAUSES  OF  FERTILITY  AND  STERILITY  [CHAP. 

Marl  containing  carbonate  of  lime  is  always  far  more 
valuable  than  clay;  pure  clay  is  so  little  friable,  and 
so  sterile  itself,  that  it  effects  an  improvement  only 
slowly ;  marl  not  only  ameliorates  the  texture  but 
adds  at  once  a  supply  of  carbonate  of  lime,  potash 
compounds,  and  in  some  cases  phosphoric  acid  also. 
Clay  and  marl  both  have  a  tendency  to  sink,  and 
eventually  require  renewing,  but  if  well  done  will  last 
for  thirty  to  fifty  years,  because  the  accumulation  of 
humus  and  fibrous  root-remains,  due  to  the  increased 
crops,  itself  binds  the  soil  together. 

At  the  present  day  the  need  of  marling  or  claying 
on  a  small  scale  is  often  seen  in  old  gardens, 
particularly  in  old  town  gardens  which  are  situated 
upon  gravel  soils,  initially  very  short  of  the  finer  soil 
particles.  The  constant  breaking  of  the  surface  by 
cultivation,  and  the  use  of  large  quantities  of  stable 
manure,  which  decays  and  leaves  the  soil  open,  result 
in  a  continual  washing  down  of  the  finest  particles,  until 
the  remaining  soil  loses  all  power  of  cohesion  and  of 
resisting  drought,  falling  into  a  dusty  powder  immedi- 
ately on  drying.  A  coating  of  clay  in  the  early  autumn, 
or,  better  still,  of  good  marl,  is  the  only  method  of 
giving  consistency  to  such  a  soil  and  soon  remedies 
its  worst  defects,  such  as  susceptibility  to  drought  and 
rapid  fluctuations  of  temperature,  and  tendency  to 
produce  soft  vegetation,  very  liable  to  disease. 

Reclamation  of  Peat  Land. 

One  of  the  earliest  methods  of  bringing  peat  land 
in  the  Fens  and  similar  districts  into  cultivation  was, 
to  dry  the  land  by  means  of  open  drains  and  break 
up  the  surface  with  the  breast  plough ;  the  clods  were 
then  gathered  together,  and  burnt  when  dry,  after- 
wards the  ashes  were  spread  and  a  crop  of  rape  taken. 


IX.]  RECLAMATION  OF  FEN  LAND  259 

The  fire  was  never  allowed  to  burn  too  fiercely,  the 
object  being  to  obtain  charred  residues  rather  than 
white  ashes.  The  effect  of  burning  the  peat  was  to 
provide  a  certain  amount  of  ash  rich  in  saline  matters 
and  particularly  in  alkaline  carbonates,  thus  correcting 
the  two  great  faults  of  the  remaining  peat,  its  deficiency 
in  mineral  matters,  and  its  sour  reaction.  At  the  same 
time  the  weeds  and  other  coarse  vegetation  occupying 
the  surface  were  destroyed,  and  a  clean  seed-bed  prepared 
for  the  crop.  However,  the  process  of  burning  is  a 
very  wasteful  one,  involving  the  loss  of  the  combined 
nitrogen  contained  in  the  accumulated  organic  matter, 
and  after  a  few  repetitions  the  land  was  found  to  be 
seriously  depleted  of  its  reserves  of  humus.  Burning 
became  replaced  in  the  Fens  by  a  marling  process, 
especially  where  the  peat  was  of  a  sandy  nature  ;  trenches 
were  opened  to  the  bed  of  marl  or  clay  always  found 
beneath  the  peat,  and  the  clay  thrown  out  and  spread 
at  the  rate  of  100  loads  or  so  per  acre,  the  burning 
process  being  reserved  for  the  first  reclamation,  when 
a  mass  of  surface  vegetation  had  to  be  got  rid  of.  In 
other  districts,  where  marl  is  less  available,  peat  has 
to  be  brought  into  cultivation  by  draining  the  land  with 
open  cuts,  allowing  some  considerable  time  to  elapse 
during  which  the  peat  dries,  shrinks,  and  consolidates, 
and  then  correcting  the  acidity  with  lime.  It  is  desir- 
able to  use  large  dressings  of  mineral  manures  like 
basic  slag  and  kainit  to  compensate  for  the  deficiency 
in  mineral  matter,  especially  where  the  peat  is  initially 
of  an  acid  character.  In  the  Fens  the  peat  is  some- 
times found  to  be  mild  humus  containing  lime ;  this 
does  not  respond  to  liming,  and  gives  better  crops  with 
superphosphate  than  with  basic  slag. 


26o    CAUSES  OF  FERTILITY  AND  STERILITY  [CHAP. 

Paring  and  Btirning. 

When  the  poorer  clay  soils  were  first  taken  into 
cultivation,  a  beginning  was  generally  made  by  "  paring  " 
the  surface  with  the  breast  plough,  and  "  burning "  the 
clods  as  soon  as  they  were  sufficiently  dry.  The  clods 
were  made  up  into  heaps  a  yard  or  so  in  diameter  with 
the  brushings  of  the  hedges  and  all  the  rough  surface 
vegetation,  together  with  as  much  clay  as  was  judged 
prudent.  Each  heap  was  then  allowed  to  burn  slowly 
and  char  the  clay,  without  permitting  the  heat  to  rise 
sufficiently  to  vitrify  the  clay  or  dissipate  such  valuable 
material  as  the  alkalis  of  the  ash.  The  resulting  ashes 
effected  a  great  improvement  in  the  soil :  the  clay  was 
partially  dehydrated,  or  at  least  coagulated,  thus  pro- 
viding a  certain  amount  of  coarse  material  to  ameliorate 
the  texture  ;  in  the  charred  clay  also,  some  of  the  potash 
was  rendered  more  available,  while  the  plant  residues 
provided  mineral  salts  and  alkalis  to  promote  nitrifica- 
tion. The  drawback  to  the  process  is  the  inevitable 
loss  of  nitrogen  to  the  soil ;  but  any  one  who  has 
noticed  how  freely  crops  grow  on  the  patches  of 
arable  land  where  couch  heaps  have  been  burnt  the 
season  before,  will  see  that,  for  the  time  being,  the 
fertility  of  the  soil  is  increased  by  the  process.  Other 
advantages  of  burning  lie  in  the  destruction  of  weeds 
and  insect  life  of  all  kinds,  and  although  it  has  been 
almost  wholly  discontinued  at  the  present  day,  the 
older  writers  on  agriculture  are  unanimous  as  to  its 
beneficial  effects  in  bringing  poor  clay  land  into  cultiva- 
tion. Recent  investigations  also  show  that  heating  the 
soil  to  low  temperatures,  such  as  that  of  boiling  water, 
bring  about  a  great  increase  in  its  productivity,  probably 
owing  to  a  rearrangement  of  the  bacterial  flora  of  the 
soil,  for  complete  sterilisation  is  only  effected  at  higher 


ix.]  LIMING  261 

temperatures.  The  subject  is  still  obscure,  but  these 
effects  of  heating  the  soil  may  well  be  a  factor  in  the 
value  of  such  processes. 

A  variation  on  the  old  process  of  "  clod  burning " 
consists  in  "  border  burning,"  in  which  clay  is  dug  from 
one  corner  of  the  field  and  burnt  by  means  of  the  couch 
and  other  weeds  cleaned  off  the  land,  the  hedge  trim- 
mings, etc. ;  the  burnt  clay  is  then  spread  over  the 
surface  to  improve  the  texture  of  the  soil.  Without 
doubt  the  latter  process  might  still  be  profitably  adopted 
where  heavy  clay  land  is  under  the  plough ;  if  every 
year  some  clay  were  added  to  the  fires  made  from  the 
weeds  and  hedge  trimmings,  valuable  material  for 
lightening  the  soil  would  be  obtained  without  wasting 
too  much  soil  nitrogen. 

Of  course  the  incorporation  of  any  large-grained 
material  will  improve  the  texture  of  clay  soils ;  in  some 
cases  sand  has  been  dug  and  spread  with  advantage  ; 
road  scrapings,  town  refuse,  and  even  coal  ashes  help 
to  lighten  the  soil,  though,  in  the  case  of  gardens,  coal 
ashes  should  be  avoided. 

Liming  and  Chalking. 

Of  all  the  methods  of  improving  the  soil,  other 
than  actual  manuring  or  cultivation,  none  is  more 
important  than  the  incorporation  of  lime  or  chalk. 
It  has  already  been  indicated  that  many  soils  exist, 
chiefly  clays  and  sands,  containing  less  than  I  per  cent, 
of  carbonate  of  lime  ;  on  all  such  land  liming  produces 
very  pronounced  effects,  both  on  the  physical  texture  of 
the  soil  and  on  the  character  of  the  resulting  vegetation. 

It  is  on  the  clays  and  other  strong  soils  that  lime 
produces  the  greatest  alteration  in  texture ;  its  effect 
in  coagulating  and  causing  the  finer  particles  to  form 
into  aggregates,  which  remain  loo.sely  cemented  by 


262    CAUSES  OF  FERTILITY  AND  STERILITY  [CHAP. 

the  carbonate  of  lime,  has  already  been  discussed. 
The  soil  becomes  much  less  retentive  of  water,  perco- 
lation is  increased  so  that  the  limed  land  is  drier 
and  warmer,  admits  of  cultivation  at  an  earlier  date 
in  the  spring,  and  is  far  more  friable  when  dry. 
In  fact,  the  liming  gives  a  coarser  texture  to  the  clay 
soil,  and  all  the  effects  pertaining  to  the  coarser  texture, 
such  as  diminished  capacity  for  retaining  water  and 
consequent  greater  warmth,  less  shrinkage  and  tendency 
tc  cake  on  drying,  are  all  manifest  after  the  application 
of  lime.  It  does  not,  however,  follow  that  the  crop 
will  mature  more  readily  though  the  season  is  made 
earlier  through  liming  ;  in  many  cases  in  dry  seasons 
crops  upon  clay  ripen  prematurely,  because  the  drying 
up  and  shrinkage  of  the  impervious  clay  cut  the 
roots  off  from  all  access  of  moisture.  The  liming, 
by  opening  up  the  soil  to  the  motion  of  water  by 
surface  tension,  keeps  the  plant  growing  for  a  longer 
period  ;  at  the  same  time,  the  increased  amount  of  plant 
food  rendered  available  also  tends  to  prolong  the  dura- 
tion of  growth.  On  very  light  soils  the  addition  of  lime 
acts  to  a  certain  extent  as  a  binding  material,  and  in- 
creases the  cohesion  and  water-retaining  power  of  the 
soil,  but  it  is  not  so  effective  in  this  respect  as  humus. 
Besides  its  physical  effect  upon  the  texture  of  stiff  soils, 
lime  has  a  very  powerful  chemical  effect,  liberating  freely 
the  reserves  of  plant  food  of  all  kinds  in  the  soil  and 
rendering  them  available  to  the  plant ;  so  that  on  soils 
naturally  deficient  in  carbonate  of  lime,  manures  of  all 
kinds  can  only  find  their  proper  value  if  lime  be  also 
used  from  time  to  time.  On  soils  that  have  been 
under  intensive  cultivation  for  a  long  time  immense 
reserves  of  plant  food  have  been  accumulated,  which 
only  require  the  addition  of  lime  to  bring  them  into 
action.  As  an  example  may  be  quoted  the  result  of 


IX.] 


ACTION  OF  LIME 


263 


applying  lime  to  an  old  hop  garden  at  Farnham, 
Surrey,  where  the  soil  consisted  of  an  alluvial  loam, 
very  deficient  in  carbonate  of  lime,  and  heavily  dressed 
with  organic  manures  for  many  years  previously.  The 
plots  chosen  for  comparison  received  a  complete  artificial 
manure  with  or  without  i  ton  of  lime  per  acre ;  the 
figures  for  the  crops  in  the  following  table  have  been 
reduced  to  percentages  to  eliminate  the  great  fluctua- 
tions due  to  season. 


Year. 

ARTIFICIAL  MANUKKH. 

With  Lime. 

Without  Lime. 

189$ 
1896 
1897 
1900 

100 
100 
100 
IOO 

70 
84 
80 
81 

190! 

IOO 

90 

Of  course,  as  lime  itself  supplies  no  food  to  the 
plant,  but  only  sets  in  action  the  dormant  residues 
already  present  in  the  soil,  the  forcing  of  crops  by  the 
aid  of  lime  alone  soon  results  in  the  exhaustion  of  the 
land.  Hence  the  old  saw : — 

"  Lime,  and  lime  without  manure, 
Will  make  both  land  and  farmer  poor." 

The  exact  effect  of  lime  in  promoting  fertility 
depends  upon  the  plant  food  in  question.  We  have 
already  seen  that  all  the  decay  processes  which  result 
in  the  oxidation  of  the  humus  are  promoted  by  the 
presence  of  a  base  to  combine  with  the  organic  acids 
produced  by  the  decay,  and,  in  particular,  that  the 
presence  of  an  easily  attacked  base  is  necessary  for 
nitrification.  As  a  nett  result,  the  oxidation  of  the 
humus  and  the  formation  of  nitrates  is  much  increased 


264    CAUSES  OF  FERTILITY  AND  STERILITY  [CHAP. 

by  a  dressing  of  lime,  which,  indeed,  is  the  first 
indispensable  step  towards  rendering  available  the  rich 
organic  residues  accumulated  in  a  sour  soil.  As  regards 
the  mineral  constituents,  lime  has  a  very  marked  power 
of  bringing  potash  into  a  soluble  state ;  the  double 
hydrated  silicates  of  potash  and  alumina,  etc.,  which 
result  from  the  partial  breaking  down  of  felspars  and 
are  the  sources  of  the  potash  of  our  soils,  are  de- 
composed, lime  being  substituted  for  the  potash  going 
into  solution.  It  is  a  case  of  mass  action,  where  the 
addition  of  one  soluble  constituent  to  the  soil  will 
increase  the  amount  that  goes  into  solution  of  all 
the  other  constituents  which  are  capable  of  being 
replaced  by  the  base  added ;  the  extent  of  the  action 
is  therefore  dependent  upon  the  amount  of  lime  used. 
The  fact  that  more  potash  has  been  rendered  avail- 
able in  limed  soils  is  clearly  seen  in  the  character  of 
the  vegetation,  e.g.,  in  an  increased  proportion  of 
clovers  in  the  herbage  of  pasture  or  hay  land.  The 
action  of  lime  as  a  liberator  of  potash  is  illustrated 
by  the  effect  of  a  dressing  of  chalk  applied  in  1881 
to  part  of  the  permanent  grass  plots  at  Rothamsted  ; 
by  1884  differences  began  to  be  manifest,  the  chalk 
caused  a  change  in  the  herbage  of  those  plots  which 
had  been  receiving  potash  each  year  for  twenty-five 
years  previously,  increasing  the  production  as  a  whole, 
and  particularly  increasing  the  proportion  of  leguminous 
plants  in  the  herbage.  On  the  plots,  however,  which 
had  been  receiving  no  potash,  and  therefore  contained 
no  recently  accumulated  reserves  of  this  material,  the 
chalk  had  practically  no  effect,  either  in  the  weight  or 
character  of  the  crop. 

To  some  extent  lime  seems  able  to  act  as  a  liberator 
of  phosphoric  acid  in  the  soil.  As  pointed  out  by 
Thenard,  lime  is  able  to  act  upon  the  very  insoluble 


IX.]  LIMING  265 

phosphates  of  aluminium  or  iron  which  are  present  in 
many  soils,  and,  by  converting  them  into  phosphate  of 
lime,  renders  the  phosphoric  acid  more  available  for 
the  plant. 

Besides  its  specific  actions  in  thus  rendering  more 
soluble  the  soil  constituents  which  nourish  the  plant, 
lime  exerts  a  very  beneficial  action  by  maintaining 
the  neutral  reaction  of  the  soil ;  it  neutralises  the  acids 
produced  by  the  decay  and  nitrification  (see  p.  174)  of 
the  organic  matter  in  the  soil,  or  those  due  to  the 
oxidation  of  materials  like  iron  pyrites  in  other  soils 
(see  p.  243).  Again,  as  has  been  shown  already,  it 
is  necessary  as  a  base  to  satisfy  the  requirements  of 
artificial  manures  like  sulphate  of  ammonia,  superphos- 
phate, and  kainit  (see  p.  216),  or  to  prevent  the  soil 
being  invaded  by  such  organisms  as  the  destructive 
fungus  causing  "  finger-and-toe  "  (see  p.  209).  It  must, 
however,  be  clearly  realised  that  lime  is  wanted  as  a 
base,  not  as  a  compound  of  calcium,  necessary  though 
calcium  itself  may  be  to  the  economy  of  the  plant ;  and 
that  only  carbonate  of  lime  (chalk,  limestone,  etc.)  or 
quicklime  and  slaked  lime,  which  promptly  become 
carbonate  of  lime  when  incorporated  with  the  soil,  are 
capable  of  acting  as  the  required  base.  Other  calcium 
compounds,  as  superphosphate  of  lime  or  sulphate  of 
lime  (gypsum),  or  phosphate  of  lime  in  bones,  etc.,  are 
either  acid  or  neutral,  and  do  not  supply  the  base 
required  to  effect  the  beneficial  actions  set  out  above  ; 
they  cannot  replace  lime  or  chalk — in  fact,  they  do  not 
contain  any  "  lime  "  in  the  farmer's  sense.  Unfortunately, 
it  has  been  too  often  supposed  that  the  use  of  artificial 
manures,  such  as  superphosphate  of  lime,  removed  the 
necessity  of  a  periodical  liming  of  the  soil,  and  some 
of  the  neglect  into  which  this  all-important  operation 
has  fallen  may  be  set  down  to  the  unfortunate  confusion 


266    CAUSES  Of  FERTILITY  AND  STERILITY  [CHAP. 

hanging  round  the  word  lime.  However,  as  will  have 
been  gathered  from  a  consideration  of  the  effects  of 
sulphate  of  ammonia  in  depleting  the  Woburn  soil  of 
carbonate  of  lime,  the  use  of  artificial  manures  generally 
demands  an  increased  rather  than  a  lessened  attention 
to  the  periodical  liming  of  the  land. 

The  method  of  liming  which  was  formerly  in  vogue 
consisted  in  applying  very  large  quantities  of  quicklime 
at  comparatively  long  intervals,  100  to  150  bushels 
per  acre  (  =  2  to  4  tons)  every  eight  or  ten  years,  or 
an  initial  dressing  of  100  bushels,  with  a  further  dressing 
of  50  bushels  per  acre  every  third  year.  The  reason 
for  this  interval  lies  in  the  fact  that  the  best  effects 
of  lime  are  to  be  seen  after  the  lapse  of  a  year  or 
two;  the  material  becomes  carbonate,  which,  being 
insoluble,  is  incorporated  with  the  soil  and  passes  into 
solution  as  bicarbonate  but  slowly.  The  immediate 
effect  of  lime  may  even  be  a  diminution  of  the  crop 
if  it  be  used  on  very  rich  land,  or  in  actual  contact  with 
fresh  dung ;  under  these  conditions  there  appears  to  be 
some  loss  of  ammonia  by  volatilisation.  Of  course  the 
effect  of  lime  is  not  very  persistent,  and  the  dressing 
must  be  repeated ;  as  the  farmers  say,  the  "  lime  sinks  in 
the  land,"  i.e.,  carbonate  of  lime  is  removed  from  the 
surface  soil  by  solution  as  bicarbonate. 

In  carrying  out  the  operation  of  "  liming,"  the  aim 
should  be  to  ensure  as  fine  a  division  as  possible,  so  as 
to  incorporate  the  material  intimately  with  the  soil.  In 
some  cases  the  lime  is  thrown  out  in  heaps  on  the 
stubbles  in  autumn,  and  slaked  by  pouring  on  water, 
the  hot  slaked  powder  into  which  the  quicklime  falls 
being  immediately  spread  over  the  land.  This  method 
only  answers  with  "  fat "  limes,  which  slake  and  fall 
readily  to  a  dry  powder ;  a  better  method  is  to  lay  up 
the  quicklime  in  heaps  and  cover  the  heaps  with  soil, 


ix.]  LIME  267 

in  which  case  the  lime  slakes  gradually  to  a  fine  powder 
that  can  be  spread  before  the  plough.  It  is  not  wise  to 
spread  the  quicklime  over  the  land,  as  much  of  it, 
after  slaking  and  becoming  carbonated,  remains  in 
lumps  which  cannot  be  reduced  to  a  powder. 

The  expense  of  liming  in  this  fashion  is  consider- 
able, and  as  the  action  is  not  immediate,  owing  to  the 
difficulty  of  getting  the  material  mixed  with  the  soil, 
it  is  desirable  to  replace  it,  if  possible,  by  a  cheaper 
process.  This  has  been  attained  by  the  use  of  ground 
lime,  which  is  at  the  present  time  prepared  by  most 
lime  works  for  the  use  of  builders ;  5  cwt.  of  ground 
lime  per  acre,  distributed  by  a  manure  barrow  or  by 
one  of  the  artificial  manure  distributors  now  manu- 
factured, will  be  found  more  effective  for  one  or  two 
seasons  than  ten  or  twelve  times  as  much  applied  in 
the  old-fashioned  method.  Of  course  such  a  small 
dressing  of  ground  lime  requires  renewing  more  fre- 
quently ;  but,  as  the  expense  is  comparatively  trifling, 
both  for  labour  and  material,  as  compared  with  the 
older  process,  it  may  be  hoped  that  on  many  soils  this 
all-important  operation  will  assume  its  old  prominence 
in  the  routine  of  farming. 

Considerable  differences  are  to  be  seen  in  the 
character  of  lime  made  from  the  various  calcium  car- 
bonate rocks  burnt  for  lime  in  the  British  Islands;  in 
the  main  a  distinction  may  be  drawn  between  the  white 
"  fat "  limes  made  from  the  White  Chalk,  the  Mountain 
Limestone  and  other  comparatively  pure  deposits  of 
calcium  carbonate,  and  the  "  thin  "  grey  or  stone  limes 
made  from  less  pure  and  more  argillaceous  limestones. 
The  "  fat "  limes  are  the  purer,  slake  readily  and  swell 
considerably  in  the  act,  forming  afterwards  a  bulky 
white  powder ;  the  "  poor  "  or  "  thin  "  limes  slake  with 
comparative  difficulty  and  do  not  increase  much  in 


268    CAUSES  OF  FERTILITY  AND  STERILITY  [CHAP. 

bulk.  The  "thin"  limes  partake  somewhat  of  the 
nature  of  a  cement,  setting  after  mixture  with  water, 
and  are  more  esteemed  by  builders  than  the  "fat" 
limes,  which  harden  with  extreme  slowness  and  are 
chiefly  employed  for  plastering  and  kindred  work. 
Naturally  the  "  fat "  limes  are  preferable  from  an  agri- 
cultural point  of  view,  both  for  their  purity  and  the 
finer  condition  into  which  they  fall ;  unfortunately  few 
of  the  lime  works  grind  the  white  lime  in  the  ordinary 
course  of  trade,  as  they  do  the  builders'  lime. 

The  lime  made  by  burning  the  Magnesian  Lime- 
stone which  occurs  in  Durham,  Yorkshire,  Derby- 
shire, and  Notts,  is  disliked  by  farmers  and  regarded 
as  injurious  rather  than  beneficial  to  the  land.  It 
contains  50  to  80  per  cent,  of  lime  and  4  to  40  per 
cent,  of  magnesia,  which  latter  constituent  may  be 
the  cause  of  the  ill  effects. 

The  following  analyses  show  the  mean  composition 
of  several  samples  of  "  fat "  and  "  poor "  lime,  being 
"white"  and  "grey"  lime  respectively,  made  from  the 
Upper  and  Lower  Chalk  of  the  North  Downs  : — 


White  Lime. 

Grey  Lime. 

Caustic  Lime 

90.20 

74-00 

Carbonate  of  Lime 

2-40 

2-66 

Magnesia 

o-35 

0-38 

Oxide  of  Iron 

0-52 

I-OO 

Alumina 

1-70 

7  -60 

Silica  as  Soluble  Silicates 

2-60 

8-60 

Insoluble  Residue 

•25 

•94 

Water,  Alkalis,  etc. 

1-98 

4-82 

100-00 

100-00 

In   place   of  lime,  chalk  may  often    be   used   with 
advantage  when  it  is  readily  accessible  ;  for  example,  on 


IX.]  CHALKING  369 

one  side  of  the  Chalk  formation  the  Gault  and  the  upper 
beds  of  the  Lower  Greensand,  and  on  the  other  side  the 
London  Clay  and  the  Bagshot  Sands,  are  generally  in 
need  of  lime,  and  are  never  very  remote  from  the  out- 
crop of  the  chalk.  The  superficial  clays  and  sands 
lying  on  the  Chalk  itself  are  often  deficient  in  lime,  and 
may  be  readily  chalked  by  sinking  shallow  pits.  It 
must  be  remembered  that  much  larger  quantities  of 
chalk  than  of  lime  are  needed  to  produce  a  given  effect ; 
not  only  is  the  chalk  equivalent  chemically  to  about 
half  its  weight  of  lime,  but  in  practice  it  can  never  be 
reduced  to  so  fine  a  state  of  division  as  lime  obtains  by 
careful  slaking.  In  chalking,  it  is  desirable  to  obtain 
the  soft  upper  white  chalk  from  a  pit,  so  that  it  is 
saturated  with  quarry  water ;  if  then  spread  over  the  land 
in  autumn  it  gets  frozen  while  still  full  of  water,  and 
becomes  reduced  to  a  comparatively  fine  powder  which 
can  be  ploughed  in  if  on  arable  soil  or  spread  with 
a  harrow  on  the  pastures.  Very  large  quantities  of 
chalk  are  used,  up  to  100  loads  to  the  acre;  naturally 
the  effect  of  such  treatment  is  more  permanent  than 
the  usual  liming. 

The  custom  of  "chalking"  was  very  extensively 
practised  during  the  seventeenth  and  eighteenth  centuries 
in  Hertfordshire  on  the  high  plateau  land  on  which  the 
Rothamsted  estate  is  situated.  There  the  "clay  with 
flints "  and  the  "  boulder  clay,"  though  not,  as  a  rule, 
more  than  10  to  12  feet  thick,  and  resting  on  the  chalk 
rock  from  which  they  have  largely  been  derived,  have 
been  completely  decalcified  by  the  solvent  action  of  the 
rain  water,  and  no  longer  contain  more  than  a  trace  of 
carbonate  of  lime.  It  was  customary  to  sink  bell  pits 
through  the  clay  until  the  chalk  was  reached  ;  this  was 
then  dug  out,  hauled  to  the  surface  in  baskets,  and  dragged 
out  on  to  the  fields  in  sledges.  Sixty  to  a  hundred  or 


270  CAUSES  OF  FERTILITY  AND  STERILITY  [CHAP.  IX. 

even  a  hundred  and  fifty  loads  per  acre  were  spread, 
and  from  time  to  time  the  process  was  repeated.  The 
amount  of  chalk  thus  spread  upon  the  surface  was  con- 
siderable ;  the  surface  soil  of  the  arable  fields  on  the 
Rothamsted  estate  now  contains  from  3  to  5  per  cent, 
of  carbonate  of  lime,  which  is  equivalent  to  30  to  50  tons 
per  acre ;  and  since  none  has  been  spread  for  the  last 
seventy  years  at  least,  and  solution  in  the  rain  water  has 
constantly  been  going  on,  there  must  have  been  nearer 
100  tons  per  acre  at  the  beginning  of  the  nineteenth 
century. 

The  result  has  been  practically  the  creation  of  a  soil 
fit  for  arable  farming,  for  some  of  the  Rothamsted  fields 
which  had  never  undergone  the  operation  have  had  to 
be  laid  down  to  grass,  so  difficult  did  their  cultivation 
prove  in  wet  seasons. 

Chalk  is  perhaps  more  suited  than  lime  to  very 
light  sandy  soils  like  the  Lower  Greensand  or  the 
Bagshot  beds,  for  on  such  dry  hot  soils  the  application 
of  quicklime  is  apt  to  result  in  too  rapid  a  decay  of  the 
organic  reserves  of  the  land ;  on  clay  soils,  however, 
quicklime  is  preferable,  as  it  is  a  much  more  effective 
agent  in  coagulating  and  improving  the  texture  of  the 
clay. 


CHAPTER   X 

SOIL  TYPES 

Classification  of  Soils  according  to  their  Physical  or  Chemical 
Nature — Geological  Origin  the  Basis  of  Classification — Vege- 
tation Characteristic  of  Various  Soil  Types  :  Physical  Structure, 
Chemical  Composition,  Natural  Flora  and  Weeds  character- 
istic of  Sands,  Loams,  Calcareous  Soils,  Clays,  Peat,  Marsh, 
and  Salt  Soils — Soil  Surveys,  their  Execution  and  Application. 

PERHAPS  the  question  of  the  greatest  practical  import- 
ance in  connection  with  the  scientific  study  of  soils 
is  their  classification  into  certain  types  defined  by 
their  physical  or  chemical  properties,  and  the  alloca- 
tion of  these  types  to  their  appropriate  areas,  so  as  to 
obtain  a  soil  map  of  any  given  district.  Despite 
disturbing  factors,  to  which  allusion  will  be  made  later, 
certain  types  of  soil  persist  over  wide  stretches  of 
country,  and  are  characterised  not  only  by  a  general 
resemblance  in  chemical  or  physical  constitution,  but 
by  a  corresponding  similarity  in  the  natural  flora  they 
bear,  and  their  appropriateness  to  certain  crops.  The 
constancy  of  the  soil  types  is  the  result  of  a  common 
origin  from  the  same  kind  of  rock,  and  the  difficulty 
lies  less  in  recognising  the  types  than  in  drawing 
boundary  lines,  so  imperceptibly  does  one  class  shade 
off  into  another.  The  only  classification  that  can  be 
at  all  general,  is  one  based  upon  the  physical  structure 

271 


272  SOIL  TYPES  [CHAP. 

and  texture  of  the  soil,  viz.,  into  sands,  loams,  and  clays, 
with  subdivisions  dependent  on  me"presence  or  absence 
of  calcium  carbonate,  and  upon  the  situation,  as  causing 
the  accumulation  or  otherwise  of  humus. 

In  attempting  to  review  the  vegetation  appropriate 
to  different  types  of  soil  it  will  be  found  that  two 
distinct  factors  must  be  taken  into  account  —  the 
relations  of  the  soil  to  water,  and  its  chemical  con- 
stitution— which  factors  often  interact  in  a  complex 
fashion,  different  causes  producing  the  same  effect.  A 
plant,  for  example,  may  be  found  upon  sand  because 
of  its  dryness,  or,  because  of  the  absence  of  calcium 
carbonate  usually  associated  with  sand ;  another 
plant,  having  adapted  its  structure  to  use  very  small 
quantities  of  water,  may  equally  well  be  found  on  a 
dry  sand,  or  on  a  clay  which  holds  so  much  water 
as  to  be  injurious  to  the  ordinary  plant. 

Plants  have  adapted  themselves  to  conditions  of 
dryness  in  very  diverse  ways ;  in  some  cases,  as  in 
gorse  or  broom,  the  leaf  surface  is  much  restricted ;  in 
others,  the  thickness  of  the  cuticle  has  been  increased, 
or  the  surface  of  the  leaf  is  thickly  clad  with  hairs ;  in 
other  cases  the  leaves  possess  special  tissues  for  storing 
water.  Such  plants  are  known  as  "  xerophytes,"  and 
are  found  on  soils  which  appear  to  differ  very  much 
from  one  another,  for  a  soil  may  contain  plenty  of 
water  and  yet  be  physiologically  dry,  because  of  the 
presence  of  some  other  constituent  hindering  the 
absorption  of  water  by  the  plant. 

The  areas  on  which  xerophytic  plants  are  found 
include  not  only  the  true  desert  areas,  where  great 
heat  and  intense  illumination  occur  during  the  larger 
part  of  the  year,  but  also  the  pervious  sandy  soils  re- 
taining very  little  moisture — sand  dunes,  shingle  flats, 
and  the  like.  Again,  the  plants  of  alkali  soils  and  of 


X.]  PHYSIOLOGICAL  DRYNESS  373 

salt  marshes  invaded  by  the  sea,  develop  a  xerophytic 
structure,  because  they  would  be  injured  if  they  absorbed 
large  amounts  of  saline  soil  water.  Peaty  areas  also 
act  in  the  same  way,  for  it  is  found  that  the  humic  acids 
in  such  soils  withhold  the  water  from  the  plant  very 
obstinately.  Exposed  elevated  regions  with  a  low 
temperature,  by  reducing  the  power  of  the  roots  to 
absorb  moisture,  render  it  necessary  that  the  plant 
should  lose  little  by  transpiration ;  hence  we  see 
certain  conifers  flourish  both  on  dry  sandy  soils  and 
wet  elevated  moors. 

As  regards  the  chemical  side  of  the  question,  the 
most  important  soil  constituent  affecting  vegetation  is 
calcium  carbonate ;  a  large  number  of  plants  seem 
absolutely  intolerant  of  lime  in  the  soil,  while  others 
are  rarely  seen  off  limestone  and  chalk  areas.  Even 
among  the  humus-loving  plants  a  different  flora  is  found 
on  the  acid  peaty  areas  from  that  prevalent  on  the 
mild  humus  areas  where  the  soil  water  contains  calcium 
bicarbonate  in  solution. 

But,  however  characteristic  the  general  aspect  of 
the  vegetation  may  be  upon  the  different  types  of  soil, 
it  is  rare  to  find  cases  of  plants  entirely  intolerant  of 
a  different  kind  of  soil  from  that  which  they  habitually 
frequent ;  many  plants  show  a  preference  for  one  soil 
or  other  without  being  exclusively  confined  to  it.  For 
example,  the  common  primrose  is  undoubtedly  a  clay 
lover,  yet  it  will  be  found  widely  distributed  over  all 
the  English  soils ;  the  beech  and  the  yew  are,  typical 
trees  of  the  chalk,  good  oak  and  hornbeam  of  the  clay  ; 
Spanish  chestnut,  and  many  conifers  like  the  Scotch 
fir,  are  sand  lovers  ;  yet  each  of  these  trees  will  be 
found  commonly  enough  on  other  kinds  of  soils.  It  will 
rarely  be  found  that  plants  will  absolutely  refuse  to  grow 
or  even  to  flourish  on  soils  of  which  they  are  naturally 

S 


274  SOIL  TYPES  [CHAP. 

intolerant ;  a  lime-hating  plant  like  gorse,  for  example, 
will  grow  freely  enough  on  a  calcareous  soil  in  a  garden 
where  it  is  protected  from  competition.  But  in  nature 
all  plants  are  subjected  to  severe  competition,  and  a  very 
small  depression  of  their  vitality  brought  about  by  the 
presence  or  absence  of  some  constituent  of  the  soil,  may 
so  turn  the  scale  against  them  that  they  are  almost 
invariably  crowded  off  areas  of  such  soil,  the  exceptions 
being  due  to  some  other  favourable  factor  coming  into 
play  locally. 

Sands. 

The  typical  sandy  soils  of  this  country  are  either 
alluvial  flats  in  the  lower  levels  of  our  rivers,  passing 
into  dunes  where  the  sand  accumulates  near  the  sea, 
or  are  directly  derived  from  some  of  the  many  coarse- 
grained sandy  formations  developed  in  England.  The 
Bagshot  beds  and  the  Lower  Greensand  form  wide 
areas  in  the  south-east ;  the  sandy  beds  of  the  Oolite 
produce  similar  soils  in  Northamptonshire  and  the 
East  Midlands ;  further  west  and  northward  the  Bunter 
beds  give  rise  to  other  very  coarse-textured  soils,  as 
does  the  Millstone  Grit  in  more  elevated  areas  in  the 
North. 

As  these  coarse-grained  sands  have  been  laid  down 
in  rough  water,  they  consist  in  the  main  of  silica,  which 
alone  is  able  to  resist  the  degree  of  weathering  and 
attrition  to  which  the  original  material  has  been 
subjected.  In  consequence,  the  rock  is  initially  with- 
out much  calcium  carbonate  or  other  material  which 
will  yield  soluble  salts  on  further  weathering ;  the  open 
texture  of  the  material  also  results  in  a  very  free 
movement  of  soil  water,  and  this  continues  the  removal 
of  anything  soluble.  Occasionally  a  sandy  rock  is 
found  which  has  been  largely  formed  by  the  disin- 


x.]  SANDY  SOILS  275 

tegration  of  shells,  so  that  it  is  rich  in  calcium 
carbonate ;  but,  as  a  rule,  sandy  soils  are  characterised 
by  poverty  in  this  material.  Most  soils  of  this  sandy 
type  seem  to  possess  considerable  amounts  of  oxide 
of  iron ;  the  actual  proportion  present  may  not  be  so 
large  as  in  ordinary  soils,  but,  being  spread  over  the 
comparatively  small  surface  offered  by  the  large  grains, 
it  is  more  in  evidence.  The  phosphoric  acid,  which 
is  rarely  present  in  any  quantity,  generally  about  CM 
per  cent,  is  chiefly  combined  with  the  oxides  of  iron. 
Because  of  the  general  lack  of  finer  particles — in  the 
main  clay  derived  from  the  weathering  of  felspar- 
soils  of  this  type  are  notably  deficient  in  potash. 

Despite  their  warmth  and  free  aeration,  sandy 
soils  often  accumulate  considerable  amounts  of  humus, 
an  effect  probably  due  to  the  absence  of  calcium 
carbonate.  Where  depressions  occur  in  the  general 
level  of  the  ground,  a  layer  of  impervious  ferric 
hydrate  or  "  pan "  forms  below  the  surface  and  holds 
up  the  drainage  water,  which  waterlogged  condition 
is  at  once  followed  by  an  accumulation  of  peat. 

On  these  sandy  areas  cultivation  is  a  very  artificial 
affair,  and  the  soil  has  practically  to  be  created.  The 
first  necessity  is  a  supply  of  lime  and  mineral  salts, 
to  remedy  the  lack  of  nutriment ;  then  as  much  humus 
as  possible  must  be  obtained,  by  turning  in  or  fold- 
ing green  crops,  or  even  from  their  roots  and  stubble 
only.  The  humus  binds  the  soil  together,  creates  a 
reserve  of  manurial  material,  and  much  increases  the 
retentive  power  of  the  soil  both  for  water  and  mineral 
salts. 

Being  so  dry,  the  specific  heat  of  sandy  soils  is 
exceptionally  low ;  in  consequence  these  soils  are 
early,  and  as  they  also  recover  quickly  from  rain,  so 
that  cultivation  is  not  forced  to  wait  much  on  the 


276  SOIL  TYPES  [CHAP. 

season  but  can  be  proceeded  with  very  readily,  they  are 
especially  suited  to  market  gardening,  wherever  situ- 
ated sufficiently  near  to  a  large  town  to  enable  large 
quantities  of  manure  to  be  obtained  cheaply.  Where 
the  water  table  is  close  to  the  surface,  sandy  soils  can 
become  very  fertile,  roots  range  freely  in  them,  and  appli- 
cations of  manure  have  their  full  effect.  Like  all  light 
soils,  they  are  apt  to  become  very  weedy.  Of  the  crops 
suitable  to  soils  of  this  type,  spring  wheat  is  often  better 
than  the  autumn-sown  variety ;  the  quality  of  wheat 
is,  however,  generally  inferior  on  sandy  soils ;  barley 
is  better  than  oats,  and  maize  is  worthy  of  attention 
as  a  fodder  crop.  Swedes,  cabbages,  and  the  cruciferous 
crops  generally,  are  subject  to  "  finger-and-toe,"  in  conse- 
quence of  the  poverty  of  the  soil  in  lime  and  soluble 
mineral  constituents. 

With  certain  exceptions,  leguminous  plants  do 
not  grow  well  on  sandy  soils,  and  require  considerable 
supplies  of  lime  and  mineral  manures ;  there  are,  how- 
ever, some  leguminous  plants  which  are  characteristically 
calcifuges  —  i.e.,  intolerant  of  lime  in  the  soil — lupins, 
serradella,  and  gorse  belong  to  this  class.  Allusion  has 
already  been  made  to  the  reclamation  of  sandy  land  in 
Prussia  by  means  of  lupins,  and  probably  more  use 
might  be  made  of  the  crop  in  this  country  on  similar 
soils.  Experiments  made  with  gorse  on  the  coarse 
sandy  soil  of  the  Royal  Agricultural  Society's  farm 
at  Woburn,  indicate  that  it  may  become  a  profitable 
fodder  crop  on  such  soils. 

Potatoes  are  perhaps  the  best  crop  on  the  sandy 
soils,  but  require  considerable  expenditure  of  manure, 
including  large  dressings  of  potash,  to  do  well.  Carrots 
are  another  crop  particularly  appropriate  to  sandy  soils, 
as  they  need  a  deep,  fine  tilth. 

The  manuring  of  sandy  soils  must  be  based  upon  a 


X.]  SANDY  SOILS  277 

liberal  use  of  lime,  frequently  renewed  because  of  the 
ease  with  which  water  percolates  and  removes  the 
calcium  carbonate.  Marling  and  chalking,  wherever 
such  materials  are  available,  are  better  for  the  land 
than  the  use  of  quicklime,  which  is  apt  to  induce  too 
rapid  an  oxidation  of  the  organic  matter.  Nitrogen  is 
best  supplied  in  its  organic  forms,  as  in  well-rotted  dung, 
the  guanos,  fish  or  meat  manure,  rape  cake,  etc. ;  nitrate 
of  soda  is  apt  to  induce  too  rapid  a  growth,  and  also  to 
be  washed  away.  Sulphate  of  ammonia  is  unsuitable, 
owing  to  the  lack  of  lime  in  the  soil.  Of  phosphatic 
manures,  superphosphate  is  unsuitable  owing  to  its 
acid  nature.  Basic  slag  is  also  unsuitable  as  a  rule, 
owing  to  the  small  quantities  of  water  retained  by 
the  soil,  but  it  answers  well  on  sands  where  the 
water  table  is  near  the  surface ;  on  the  whole, 
neutral  easily  available  phosphates  like  phosphatic 
guano  and  steamed  bone  flour  give  the  best  results 
on  these  soils.  Potash  manures  are  much  needed, 
and  either  kainit  or  sulphate  of  potash  may  be  used. 
Gypsum  is  often  used  with  good  effect  on  such  soils 
in  the  Wealden  area,  acting  as  a  liberator  of  what 
little  potash  may  be  in  the  soil. 

The  natural  flora  of  the  sandy  soils  is  of  a  double 
character — in  part  xerophytic,  and  associated  with  the 
prevailing  dryness  of  the  soils ;  in  part  calcifuge, 
and  dependent  on  the  absence  of  calcium  carbonate. 
Plants  with  mycorhiza  are  abundant,  owing,  as  already 
explained,  to  the  comparative  poverty  of  these  soils 
in  both  water  and  soluble  salts. 

The  characteristic  sand  trees  are  the  Spanish 
chestnut,  birch,  holly,  and  many  conifers ;  of  these 
the  Spanish  chestnut  and  some  of  the  firs,  like 
Pinus  pinaster,  are  particularly  intolerant  of  calcium 
carbonate. 


278  SOIL  TYPES  [CHAP. 

Many  of  the  Ericaceae,  such  as  common  heather  and 
the  heaths,  cultivated  species  like  the  rhododendrons 
and  azaleas  of  our  gardens,  are  similarly  intolerant  of 
lime  and  associated  with  sandy  soils ;  at  higher  levels 
various  species  of  Vaccinium  and  kindred  plants  are 
common.  Gorse  (Ulex  europaus  and  U.  nanus\  broom 
(Cytisus  scoparius),  Genista  anglica,  Ornithopus,  and 
several  vetches  like  Vicia  cracca,  are  characteristic  legu- 
minous plants  of  sandy  soils.  The  foxglove  (Digitalis 
purpured),  sorrel  (Rumex  acetosella),  and  in  undrained 
situations  the  sundews  (Drosera  sp.)  are  intolerant  of 
lime  and  are  common  plants  on  sandy  soils,  as  also 
are  the  common  bracken  (Pteris  aquilina\  and  wavy  hair 
grass  (Aira  flexuosa). 

Characteristic  weeds  of  sandy  soils  are,  spurrey 
(Spergula  arvensts),  and  sandwort-spurrey  (Spergularia 
rubra\  corn  marigold  (Chrysanthemum  segetum),  and 
knawel  (Scleranthus  annuus  and  perennis) ;  Papaver 
dubium  and  Centaurea  cyanus  are  also  common  on 
such  soils.  The  bulbous  buttercup  (Ranunculus  bul- 
bosus}  is  very  frequent  on  dry  pastures,  whether  sandy 
or  chalky,  as  is  the  small  bindweed  (Convolvulus 
arvensis)  of  similar  soils  under  cultivation ;  the  silver- 
weed  (Potentilla  anserina\  though  generally  indicative 
of  winter  flooding,  is  to  be  found  on  all  kinds  of 
poor,  light  land. 

The  Loams. 

The  sandy  soils  pass  by  imperceptible  stages  into 
the  loams — free-working  soils  containing  enough  sand 
to  be  friable  and  to  admit  of  percolation,  yet  retain- 
ing sufficient  water  near  the  surface  to  withstand 
short  spells  of  dry  weather.  If  the  sandy  fractions  of 
the  loam  are  mainly  fine  grained,  the  soil  is  apt  to 
run  and  become  very  sticky  in  wet  weather,  afterwards 


x.]  ALLUVIAL  LOAMS  279 

drying  to  hard  clods ;  an  admixture  of  coarser  sand 
results  in  a  better  texture.  The  loams  are  typical 
soils  of  arable  cultivation  and  are  suitable  to  all  crops  ; 
their  manurial  requirements  vary  with  the  origin  of 
each  soil,  and  are  largely  conditioned  by  its  poverty 
or  richness  in  calcium  carbonate.  While  no  special 
flora  can  be  associated  with  the  loams,  there  are  several 
weeds  generally  taken  as  indicative  of  good  fertile  soils 
of  this  class ;  such  are  chickweed  (Stellaria  media), 
groundsel  (Senecio  vulgaris},  fat  hen  (Chenopodium 
album},  stinking  mayweed  (Anthemis  Cotula),  and 
the  Sow  thistle  (Sonchus  oleraceus).  Other  weeds 
of  cultivated  land,  which  only  occur  when  the  soil 
is  capable  of  carrying  fair  crops,  are  goose  grass 
(Galium  aparine\  the  speedwells  ( Veronica  agrestis, 
etc.),  pimpernel  (Anagallis  arvensis),  henbit  (Lemium 
amplexicaule\  wild  poppy  {Papaver  rhaeas},  and  the 
small  spurges  like  Euphorbia  Peplus.  The  alluvial 
soils  which  border  the  rivers  and  pass  into  con- 
siderable marshes  at  their  mouths,  must,  by  their 
texture,  be  classed  among  the  loams,  and  present  no 
specific  features,  except  where  they  are  waterlogged 
and  marshy,  or  near  the  sea  where  the  subsoil  water 
becomes  so  rich  in  salt  as  to  alter  the  character  of 
the  vegetation.  The  marshy  patches  accumulate,  as 
a  rule,  what  has  already  been  described  as  "mild 
humus,"  owing  to  the  presence  of  bicarbonate  of  lime 
in  the  soil  water ;  it  is  generally  accompanied  by 
deposits  of  pulverulent  ferric  hydrate.  The  presence 
of  rushes,  of  sedges  like  the  carnation  grass,  or  orchids 
like  0.  maculata  and  O.  latifolia,  are  characteristic  of 
these  spots  requiring  drainage.  Lousewort  (Pedicularis 
palustris]  is  said  only  to  occur  in  marshes  where  the 
water  contains  lime. 

The   salt   marshes    possess   a   characteristic  vegeta- 


280  SOIL  TYPES  [CHAP. 

tion  of  what  are  termed  halophytes,  plants  capable 
of  resisting  a  considerable  quantity  of  salt  in  the 
medium  in  which  they  grow.  Among  cultivated 
plants,  mangolds,  asparagus,  and  crucifers  like  cabbage, 
are  most  tolerant  of  salt,  and  the  two  former  are  true 
halophytes. 

Many  halophytes  live  by  acting  as  xerophytes,  and 
taking  very  little  water  up ;  they  are  also  able  to 
store  away  in  their  tissues  quantities  of  saline  matter 
which  would  be  toxic  to  the  majority  of  plants.  The 
ash  of  Armeria  maritima  shows  12  to  15  per  cent,  of 
chlorine  in  the  ash ;  in  Aster  tripolium  the  proportion 
rises  to  over  40  per  cent,  in  the  ash  of  the  leaves,  and 
to  50  per  cent,  in  that  of  the  stem ;  yet  although  the 
plants  habitually  contain  these  large  amounts  of  salt, 
they  will  grow  perfectly  well  in  ordinary  soil  where 
they  can  get  but  little. 

The  Australian  salt-bush  (A  triplex  semibaccatum}, 
which  has  already  been  mentioned  as  tolerant  of  a 
large  amount  of  alkali  in  the  soil,  also  removes  much 
soluble  matter — the  dry  plant  containing  as  much  as 
20  per  cent,  of  ash,  so  that  the  salt  content  of 
the  soil  may  be  materially  reduced  by  cropping  with 
this  plant.  The  halophytes  seen  in  the  salt  marshes 
of  this  country  consist  of  various  species  of  A  triplex, 
Beta  (the  source  of  the  cultivated  beets  and  man- 
golds), and  other  Chenopodiaceae,  Statice  armeria, 
Aster  tripolium,  Frankenia,  and  a  number  of  crucifer- 
ous plants  like  Crambe,  and  Cakile,  with  umbellifers 
like  Crithmum.  Some  plants  show  a  great  dislike  to 
salt,  even  in  small  proportion,  e.g.,  the  Rosaceae, 
Orchidaceae,  and  the  Ericaceae. 


x.]  CALCAREOUS  SOILS  381 

Calcareous  Soils. 

It  is  difficult  to  draw  an  exact  line  of  demarcation 
between  the  loams  and  calcareous  soils,  so  variable  is 
the  proportion  of  carbonate  of  lime,  owing  to  its  con- 
tinual removal  by  the  percolation  of  water  containing 
carbonic  acid.     Even  on  the  chalk  and  limestone,  where 
the  thickness  of  the  soil  layer  is  to   be  measured    in 
inches,  the  surface  soil  may  have  its  calcium  carbonate 
almost  wholly   removed,  and,  again,  where  the  deeper 
soils   of   calcareous   origin   accumulate   in   the   valleys, 
there    is    nothing   to   distinguish   them    from   ordinary 
loams.     However,  the  general  aspect  of  the  calcareous 
soils   containing   from    5    to   60   per    cent,    of   calcium 
carbonate  is  characteristic,  and  the  natural  flora  always 
indicates  the  presence  of  much  lime.     The  texture  of 
the  calcareous  soils  may  vary  within  any  limits,  accord- 
ing to  the  formation  from  which  they  have  originated. 
On  the  one  hand,  extremely  fine-textured  heavy  marls 
exist ;    for  example,  the  soils  derived   from  the  strata 
at  the  base  of  the  Chalk  and  upper  beds  of  the  Gault 
in  the  south  and   east   of  England ;    on  the  contrary, 
fairly  coarse  sand  may  form  a  considerable  proportion 
of  the  soil,  rendering  it  light  in  texture,  as  is  the  case 
with    many  of  the  soils   resting   on   the   chalk    of  the 
North  Downs.     In  all  cases  these  calcareous  soils  are 
typically  sticky  when  wet,  and  easily  cake  on  the  surface 
when  dried.     Such  soils,  again,  lose  their  organic  matter 
very  rapidly  by  decay ;  in  farming  them  it  is  desirable 
to  use  every  means  to  increase  the  proportion  of  humus 
by  adding  farmyard   manure,  by  folding  roots  on  the 
land,  or   by  ploughing  in   green   crops.     Slowly  acting 
nitrogenous    manures,   like   rape    dust   or   shoddy,   are 
valuable ;      again,    there     is    always    enough    calcium 
carbonate   present   naturally   to   render   to   sulphate  of 


282  SOIL  TYPES  [CHAP. 

ammonia  its  full  value  as  a  source  of  nitrogen.  The 
lighter  calcareous  soils  require  a  free  use  of  nitrogenous 
manures  to  get  good  crops.  Calcareous  soils  are 
generally  well  provided  with  phosphoric  acid,  owing  to 
the  organic  origin  of  the  calcium  carbonate ;  the  rule 
is,  however,  by  no  means  universal,  the  upper  Chalk, 
for  example,  yields  soils  with  less  than  01  per  cent, 
of  this  constituent.  Superphosphate  is  undoubtedly 
the  best  source  of  phosphoric  acid  for  such  soils, 
basic  slag  is  almost  without  action.  The  propor- 
tion of  potash  present  in  these  soils  is  generally 
reflected  in  their  texture ;  if  light  and  near  the 
unaltered  rock,  they  are  as  a  rule  very  deficient  in  this 
constituent,  and  require  its  addition  for  the  growth 
of  any  of  the  root  crops.  Salt  is  generally  bene- 
ficial as  an  addition  to  manures  on  the  calcareous 
soils. 

The  calcareous  soils  are  generally  warm,  dry,  and 
healthy  for  stock ;  when  deep  and  sheltered  they  are 
extremely  fertile ;  the  thinner  soils  are  rather  subject 
to  certain  insect  pests,  like  the  turnip  flea.  The  abund- 
ance of  worms  in  chalky  pasture  is  worthy  of  note. 
The  lighter  calcareous  soils  are  notoriously  weedy.  In 
addition  to  the  usual  weeds  of  light  land,  Fumitory 
(Fumaria  officinalis],  Geranium  molle,  and  kindred 
species,  are  almost  confined  to  soils  with  a  consider- 
able proportion  of  calcium  carbonate.  Two  crops 
are  very  characteristic  of  calcareous  soils  wherever 
the  climate  will  admit  of  their  growth,  viz.,  sainfoin 
and  lucerne,  which  flourish  excellently,  and  provide 
abundant  and  valuable  fodder  even  on  the  driest 
chalk  soils. 

The  natural  flora  of  these  calcareous  soils  includes 
the  beech,  yew,  and  wild  cherry,  among  trees ;  the 
juniper,  box,  mealy  guelder  rose  ( Viburnum  lantana), 


X.]  CALCAREOUS  SOILS  283 

beam  tree  (Pyrus  aria\  dogwood  (Conius  sanguinea), 
and  Clematis  vitalba,  among  shrubs. 

The  vegetation  is  characteristically  rich  in  flower- 
ing plants:  amongst  the  Leguminosae,  the  horse -shoe 
vetch  (Hippocrepis  coinosa),  bird's-foot  trefoil  (Lotus 
corniculatus),  kidney  vetch  (Ant hy  His  Vulneraria},  are 
everywhere  abundant ;  milkwort  (Poly gala},  bladder 
campion  (Silene  inflatd},  Spircca  filipendnla,  burnet 
(Poterium  sanguisorba),  wild  parsnip  (Pastinaca  satii<a), 
sheep's  scabious  (Scabiosa  columbaria],  chicory  (Cicho- 
rium  intybus},  and  certain  of  the  Gentianaceae,  as  G. 
amarella  and  Chlora  perfoliata,  the  viper's  bugloss 
(Echium  vulgare},  and  a  number  of  labiates  like  Ori- 
ganum, are  characteristic  of  the  pastures  and  waste 
places  on  chalk  and  limestone.  Amongst  grasses, 
Avena  pubescens,  A.  flavescens,  Bromus  erectus,  and 
Brachypodium  pinnatum,  are  common. 

While  it  has  been  indicated  that  many  plants  are 
intolerant  of  lime,  others  show  the  effect  of  any  excess 
in  the  soils  by  a  stunted  development  of  the  plant, 
often  accompanied  by  a  reduced  size  of  the  leaf,  and 
a  sickly  yellow  or  even  white  colour.  This  unhealthy 
condition  of  "chlorosis"  is  particularly  noticeable  on 
the  stiff  marls,  which  are  but  little  aerated  but  contain 
much  calcium  carbonate ;  on  the  Continent  it  often 
affects  vines,  particularly  when  grafted  on  American 
stocks. 

Clay  Soils. 

It  has  already  been  indicated  that  clay  soils  are 
those  in  which  the  finer  fractions  of  sand,  silt,  and 
clay  predominate  ;  the  presence  of  any  considerable  pro- 
portion of  coarse  sand  causes  the  soil  to  become  friable, 
and  would  class  it  with  the  loams.  The  texture  of 


2«4  SOIL  TYPES  [CHAP. 

clay  soils  naturally  varies  very  much ;  the  heaviest 
clays  occur  in  dry  climates,  where  the  percolation 
has  not  been  sufficiently  great  to  wash  away  many 
of  the  finer  particles ;  in  the  east  and  south-east  of 
England  the  Oxford  and  the  London  Clay,  with  the 
Boulder  clays  derived  therefrom,  give  rise  to  the 
most  stubborn  and  intractable  clays.  On  these  soils 
the  old  practice  of  an  occasional  bare  fallow  is  still 
carried  out,  and  is  almost  necessary  to  maintain  the 
soil  in  good  cultivation.  As  a  rule,  the  strong  clay 
soils  have  of  late  years  been  laid  down  to  permanent 
pasture ;  the  cost  and  the  difficulty  of  arable  culti- 
vation (for  much  wet  weather  in  autumn  or  spring 
may  render  it  impossible  to  put  horses  on  the  land 
for  long  periods),  and  the  great  fall  in  prices  of  both 
wheat  and  beans,  the  staple  crops  of  such  land,  have 
rendered  it  necessary  to  resort  to  a  cheaper  method 
of  farming. 

Most  clays  carry  good  permanent  pasture,  because 
the  soil  retains  enough  water  to  keep  the  grass  growing 
through  any  but  the  longer  periods  of  drought ;  in  very 
dry  years,  however,  clay  suffers  severely  from  the 
drought ;  the  surface  cracks  and  the  subsoil  dries 
through  the  cracks ;  the  resistance  also  offered  by  the 
close  texture  of  the  soil  to  the  capillary  rise  of  soil 
water  renders  the  winter  rainfall  less  available  to  the 
crop  than  on  soils  of  lighter  texture.  The  benefits 
accruing  from  drainage,  in  making  the  soil  dry  more 
quickly  after  rain  and  resist  drought  better,  have  already 
been  discussed.  Certain  clay  soils  may  be  found  too 
close  textured  to  carry  good  pasture ;  the  soil  sets  so 
firmly  that  aeration  becomes  very  defective,  and  the 
vegetation  degenerates  into  surface  rooting,  stoloniferous 
grasses  like  Agrostis  alba. 

Owing  to  their  fine  division,  their  origin  from  the 


X.]  CLA  Y  SOILS  285 

compound  silicates  of  primitive  rocks,  and  the  reduced 
percolation  which  they  permit,  all  clays  are  compara- 
tively rich  in  soluble  mineral  salts.  Many  of  them  show 
crystals  of  selenite  (CaSO42H2O)  in  the  subsoil  com- 
paratively near  to  the  surface ;  magnesium  sulphate  is 
often  abundant,  and  strongly  impregnates  the  water 
obtained  from  the  wells  or  the  occasional  springs  to  be 
found  in  the  clays.  In  the  Weald  of  Kent  the  shallow 
wells  in  the  clay  yield  water  that  is  almost  undrinkable, 
containing,  as  it  does,  from  150  to  450  parts  of  dissolved 
matters  per  100,000,  consisting  chiefly  of  the  sulphates 
(with  some  chlorides)  of  magnesium  and  calcium.  The 
sulphates  often  originate  from  the  oxidation  of  finely 
divided  iron  pyrites.  The  presence  of  ferrous  salts 
and  other  unoxidised  iron  compounds  has  already 
been  alluded  to  as  a  source  of  sterility  in  clay  soils, 
particularly  where  the  subsoil  has  been  incautiously 
brought  to  the  surface.  In  the  cultivation  of  all  land 
it  is  important  to  keep  the  surface  soil  on  the  top,  and 
to  attempt  to  deepen  the  staple  with  care ;  but  this  is 
particularly  the  case  with  clays,  where  the  land  may 
easily  be  injured  for  years  by  over-deep  ploughing.  No 
soils  show  more  marked  change  than  the  clays  do  in 
passing  from  soil  to  subsoil,  both  in  chemical  com- 
position and  physical  texture. 

Many  clay  soils,  especially  when  undrained,  possess 
a  great  tendency  to  accumulate  hydrated  ferric  oxide 
some  few  inches  below  the  surface,  at  about  the  level 
to  which  the  soil  is  ordinarily  aerated.  This  deposit 
sometimes  forms  a  continuous  layer  or  "  pan  "  ;  in  drier 
climates  it  becomes  a  kind  of  "  crowstone  "  gravel,  made 
up  of  little  nodules  of  hydrated  oxide  of  iron,  contain- 
ing also  manganese.  This  material  frequently  forms 
a  serious  obstacle  to  cultivation,  and  requires  to  be 
broken  up  with  a  crowbar  or  a  subsoil  plough  before 


286  SOIL  TYPES  [CHAP. 

any  deep-rooting  crop  can  be  properly  grown.  Its 
origin  is  perhaps  not  entirely  explained  as  yet ;  the 
respective  shares  of  the  iron  bacteria  of  Winogradsky, 
or  the  purely  chemical  actions  of  solution  and  reduc- 
tion by  the  organic  matter  and  carbonic  acid,  followed 
by  redeposit  on  evaporation,  is  a  matter  requiring 
further  investigation.  The  formation  of  the  material 
is  only  noticed  in  clays  very  poor  in  calcium  carbon- 
ate and  liable  to  waterlogging  through  insufficient  per- 
colation. 

Owing  to  their  coolness,  their  retention  of  moisture, 
and  comparative  impermeability  to  air,  humus  tends  to 
accumulate  in  the  clay  soils ;  both  arable  and  pasture 
soils  show  a  higher  proportion  of  organic  matter  and 
of  humus  than  is  found,  as  a  rule,  on  the  lighter 
lands ;  the  effect  of  manures  like  farmyard  manure  is 
also  more  lasting.  The  use  of  more  slowly  acting 
nitrogenous  manures  is  therefore  not  so  desirable 
on  the  clay  soils ;  on  the  other  hand,  sulphate  of 
ammonia  is  often  unsuitable  because  of  the  want  of 
calcium  carbonate,  and  nitrate  of  soda,  which  often 
gives  the  best  returns,  is  apt  to  affect  the  texture 
injuriously. 

The  clays  are  very  generally  deficient  in  calcium  car- 
bonate, often  to  an  extreme  degree,  much  to  the  detri- 
ment of  the  texture  of  the  soil.  The  use  of  lime  is  of 
the  utmost  value  to  all  clay  soils,  improving  the  texture, 
making  them  drier  and  therefore  warmer  and  earlier, 
and  rendering  available  the  supplies  of  nitrogen  and 
potash  with  which  they  are  often  liberally  endowed. 
The  excess  of  magnesia  and  unoxidised  iron  com- 
pounds which  also  characterise  many  clays  is  corrected 
by  the  use  of  lime. 

Many  clay  soils  also  show  a  considerable  deficiency 
of  phosphoric  acid,  and  respond  freely  to  dressings 


x.]  CLA  Y  SOILS  287 

with  manures  containing  this  substance.  Superphos- 
phate may  be  used  with  advantage  wherever  there  is 
enough  calcium  carbonate  in  the  soil,  but  basic  slag 
is  the  typical  phosphatic  manure  for  the  strong  soils 
which  retain  sufficient  water  to  render  the  phosphates 
active.  While  supplying  phosphoric  acid,  it  also 
contains  free  lime  in  a  fine  state  of  subdivision,  and 
so  liberates  in  a  soluble  state  the  reserves  of  nitrogen 
and  potash  in  the  soils.  It  should,  however,  not  be 
forgotten  that  as  the  basic  slag  only  supplies  one 
element  of  plant  food,  the  phosphoric  acid,  the  soil 
may  easily  be  exhausted  by  continual  cropping  and 
manuring  with  basic  slag  alone. 

Potash  is  always  present  in  large  amounts  in  clay 
soils,  0-5  per  cent,  soluble  in  strong  hydrochloric  acid 
is  often  to  be  found,  while  the  proportion  which  can 
be  extracted  after  completely  breaking  up  the  soil 
with  hydrofluoric  acid  may  rise  to  2  per  cent.  Clay 
soils  are  late,  and  their  crops  grow  slowly  and  ripen 
tardily  except  in  specially  dry  seasons,  when  the  clay 
shrinks  so  much  as  to  cut  off  all  access  of  moisture 
from  the  subsoil,  and  prematurely  ends  the  period  of 
growth ;  on  the  other  hand,  the  quality  of  crops  grown 
on  the  clay  is  often  high.  The  typical  crops  of  strong 
land  are  wheat,  beans,  and  mangolds ;  owing  to  the 
closeness  of  the  texture  of  the  soils,  weeds  are  much 
less  in  evidence  on  the  clays  than  elsewhere,  though 
some  few  are  exceedingly  troublesome. 

On  the  poorer  pastures,  the  spiny  form  of 
rest  harrow  (Ononis  arvensis),  the  wild  teazel 
(Dipsacus  sylvestris},  Ranunculus  arvensis,  and  Genista 
tinctoria,  are  characteristic  and  often  troublesome 
weeds  ;  on  cultivated  land  the  "  black-bent "  grass 
(Alopecurus  agrestis\  and  field  mint  (Mcnt/ia  an'ensis) 
are  difficult  to  deal  with.  Other  plants  characteristic 


288  SOIL  TYPES  [CHAP. 

of  strong  soils  are  the   primrose   (Primula    vulgaris], 
and  the  wild  carrot  (Daucus  carotd). 


Peaty  Soils. 

The  accumulation  of  humus  to  form  peaty  soils  has 
already  been  discussed,  and  is  associated  with  water- 
logging, which  cuts  off  the  access  of  air  and  so  sets 
up  an  anaerobic  fermentation  of  the  residues  of  the 
vegetation  growing  upon  the  surface.  There  is  always 
a  deposit  of  ferric  hydrate  accompanying  the  accumu- 
lation of  peat,  as  explained  before.  As  the  reclama- 
tion of  peaty  soils  has  already  been  dealt  with,  it  will 
be  sufficient  here  to  indicate  that  their  great  character- 
istic is  a  deficiency  in  soluble  mineral  constituents, 
notably  salts  of  lime  and  potash.  It  has  also  been 
mentioned  that,  as  a  consequence  of  the  acid  nature 
of  the  medium,  the  bacteria  of  nitrification  are  absent 
or  few  in  number.  All  attempts  at  the  cultivation 
of  peaty  soils  begin  with  drainage,  and  must  then 
proceed  on  the  basis  of  neutralising  the  organic  acids 
with  lime  and  providing  a  sufficiency  of  mineral  food 
for  the  plant,  thus  also  inducing  nitrification  to  render 
available  the  large  quantities  of  nitrogen  which  have 
accumulated.  Of  the  common  crop  plants,  oats  and 
potatoes  are  perhaps  the  most  tolerant  of  extreme 
amounts  of  acid  humus.  The  normal  vegetation  of 
peaty  soils  is  a  mixture  of  xerophytic  and  calcifuge 
forms;  the  Conifers,  the  Ericaceae,  Drosera,  Rumex 
Acetosella,  Pedicularis  sylvatica,  Sphagnum  moss,  and 
many  sedges  and  rushes,  are  characteristic  of  sour, 
peaty  soils.  Other  characteristic  plants  are  more  pro- 
perly Northern  or  Arctic  species,  and,  occurring  only  in 
the  uncultivated  uplands,  need  not  be  considered  here. 


x.]  SOIL  SURVEYS  289 

Soil  Surveys. 

To   render  the    scientific    study   of   soils    properly 
available  for  the  service  of  the  agriculturalist,  more  is 
required   than   the   examination   of  single   samples   of 
soil,  representing,  at  the  best,  only  the  land  dealt  with 
by  one  person.     Over  any  wide  district,  not  only  would 
such  work  become  expensive    and    practically  endless, 
liable   also   to    many   sources    of   error    through    local 
and  accidental  variations  of  the  soil  on  the  spot  from 
which  the  sample  was  drawn,  but  each  analysis  would 
lose  the  greater  part  of  its  value  if  it  could  not  be  co- 
ordinated and  brought  into  line  with  others  drawn  from 
soils   of  the   same   type.     A   general   soil   survey  of  a 
district,  so  as  to  be  able  to  lay   down  a    plan    of  the 
distribution  of  the  various  soil  types,  accompanied   by 
a    discussion     of    the    broad    characteristics    of    each, 
should  be  the  basis  upon  which   the   interpretation   of 
the   analysis   of  the   soil   of  any  particular  field    is   to 
be  founded.     Only  by  comparison   with   the  type   can 
the   analysis   of  any  particular   soil  be  properly  inter- 
preted— e.g.,  the   fact   that    a   soil  from  a  given  arable 
field   contains   o  1 5    per  cent,  of  phosphoric  acid  takes 
a   very   different   aspect   when    it    is   known    that    the 
soils  of  the  same  type  contain  as  a  rule  018   to  020 
per  cent,   of  phosphoric   acid,   particularly  if,  also,  the 
response  of  that  kind  of  land  to  phosphatic  manures  is 
known  by  field  trials,  or  from  the  accumulated  experience 
of  farmers.     The  first   question   which    requires   settle- 
ment  is   how   far   a   soil   survey   is   possible :   to  what 
extent  can  the  boundaries  o.L_sgjj_  types  be  traced  ;  arg 
the  various  types  sufficiently  constant  over  a  wide  area 
to  render  this  mapping  feasible?     In  many  cases  there 
seems  to  be  little  but  confusion,  even  in  the  soils  on  a 
single  farm  ;  field  differs  from  field,  and  great  variations 

T 


290  SOIL  TYPES  [CHAP. 

may  be  manifest  even  within  the  confines  of  a  single 
field.  But,  in  the  main,  each  soil  type  has  a  well-defined 
area,  within  which  it  presents  a  reasonably  constant 
composition  and  texture,  and  though  the  boundaries 
cannot  be  laid  down  with  the  precision  of  the  outcrop 
of  a  stratum,  the  zone  of  transition  from  one  type  to 
the  other  may  be  indicated  with  approximate  accuracy. 

The  basis  upon  which  any  soil  survey  must  be 
constructed  is  the_Qrigin_Q£_the  sni^;  each  geological 
formation,  for  example,  will  give  rise  to  a  distinct  type 
of  soil  if  it  has  been  formed  in  situ;  should  the 
weathered  material  have  further  undergone  transport 
by  water,  two  or  more  types  may  have  been  constructed 
by  the  sorting  action  of  the  water.  It  is  also  well 
known  that  a  geological  formation  may  change  very 
considerably  in  lithological  character  in  passing  from 
the  lower  to  the  upper  portions  of  the  bed.  For 
example,  stiff  as  the  London  Clay  is,  the  upper  beds 
become  increasingly  sandy  in  character,  so  that  it  is 
not  easy  to  draw  a  line  of  demarcation  between  the 
soil  arising  from  these  beds  and  those  due  to  the  Bag- 
shot  Sands  above.  The  lower  beds  of  the  Gault  Clay 
are  also  very  pure,  and  give  rise  to  a  stiff  clay  deficient 
in  calcium  carbonate ;  the  upper  beds  become  marly, 
and  form  soils  indistinguishable  from  those  due  to  the 
contiguous  Chalk  Marl.  Similarly,  a  geological  forma- 
tion may  show  a  progressive  change  of  character  in 
passing  into  a  different  area,  which  change  will  be 
reflected  in  the  soils  derived  from  them.  For  example, 
the  Great  Oolite  limestones  of  the  Cotswolds  shade 
off  into  sandstones  in  Northamptonshire,  and  the  Hythe 
beds  of  calcareous  sandstone  in  East  Kent  become  pure 
coarse-grained  sandstones  in  West  Surrey.  However, 
in  the  main,  geological  origin  may  be  taken  as  the 
basis  of  a  soil  survey,  to  which  must  be  added  the~ 


x.]  MAPPING  SOILS  291 

further  subdivisions  due  to  the  causes  enumerated 
above,  or  to  the  local  movements  of  rain-wash  or  ill- 
defined  drift,  that  may  alter  the  character  of  the  soil 
without  being  of  any  particular  geological  importance. 

A  soil  map  will  consist  of  a  "drift"  map,  with  some 
further  details  showing  the  superficial  formations  occupy- 
ing the  surface  of  the  ground,  and  notes  regarding  the 
local  variations  in  the  type  of  soil  derived  from  each 
particular  stratum.  The  aim  of  a  soil  survey  is  to 
carry  further  the  work  of  the  geological  survey  as 
regards  the  superficial  formations ;  the  only  classifica- 
tion  which  can  be  adapted  will  be  one  base/d  upon,  tfie 
physical  texture  of  the  soilt  arid  indicated  by  such 
conventional  terms  as  clays,  clay  loams,  sandy  loam.-, 
marls,  etc._  At  the  same  time,  the  map  must  indicate 
tfie  various  origin  of  the  different  loams  which  may 
be  found  in  the  area  under  survey,  and,  by  reference 
to  the  accompanying  text,  should  give  those  details 
of  physical  structure  and  chemical  constitution  which 
characterise  the  soil,  but  which  cannot  be  set  out 
except  by  an  over-elaborate  classification. 

The  field  portion  of  the  work  of  a  soil  survey  con- 
sists in  the  exploration  of  the  subsoil  by  means  of 
an  auger,  aided  by  any  natural  sections  which  may 
be  displayed.  The  boundary  between  two  soil  types 
may  generally  thus  be  laid  down  by  the  aspect  of  the 
soil  and  subsoil ;  from  time  to  time,  however,  samples 
must  be  retained  for  more  detailed  examination  in 
the  laboratory,  whenever  the  look  and  touch  of  the 
material  are  not  sufficient  for  a  decision  /;/  situ.  An 
immediate  examination  with  the  microscope,  the 
behaviour  of  the  soil  with  acid,  or  a  rough  sifting  in 
a  stream  of  water,  will,  as  a  rule,  be  sufficient  to 
refer  a  given  example  of  subsoil  to  one  type  or  another. 
Complicated  cases  arise  from  time  to  time,  especially  in 


392  SOIL  TYPES  [CHAP 

the  river  valleys,  where  alluvium  of  varied  epochs  and 
rain-wash  of  not  very  different  origin  may  be  hope- 
lessly intermingled.  In  many  cases,  however,  where 
the  outcrop  of  the  originating  formations  is  broad,  and 
where  the  gradients  of  the  country  are  slight,  the  soil 
may  be  extremely  constant  in  composition  over  a  wide 
area,  so  that  the  survey  has  only  to  notice  such  minor 
variations  as  the  grading  from  a  lighter  to  heavier  type 
as  one  descends  a  slope,  or  the  occasional  influx  of 
drift  material  by  creeping  from  a  neighbouring  area. 
The  further  work  of  a  field  survey  will  be  the  selection 
of  typical  samples  of  soil  and  subsoil  for  detailed 
examination  and  analysis  in  the  laboratory,  the  col- 
lection of  such  data  as  the  distance  to,  and  nature  of 
ground  water,  and  any  particulars  which  may  be 
available  locally,  as  to  special  features  in  the  working 
of  each  type  of  soil,  or  in  the  growth  of  its  crops ; 
the  nature  and  character  of  such  deposits  as  "brick 
earth"  in  each  district  can  also  be  reported.  The 
samples  for  detailed  analysis  should  be  taken  where  a 
general  survey  of  the  district  indicates  the  soil  as  most 
likely  to  be  typical  of  the  formation,  and  free  from 
admixture  with  drift  and  other  accidental  intrusions. 
At  the  same  time,  since  the  soil  in  the  main  will  by  no 
means  be  so  pure  as  the  typical  samples,  a  much  larger 
number  of  samples  should  be  taken  and  subjected  to 
a  less  detailed  examination,  by  way  of  ascertaining 
within  what  limits  the  normal  variation  of  the  soil  is 
confined. 

The  number  of  typesamples  to  be  taken  must  be 
entirely  decided  by  an  examination  of  the  circum- 
stances ;  in  continental  areas,  where  deposition  has 
been  very  uniform  over  wide  districts,  one  or  two 
samples  may  be  sufficient  to  characterise  an  extensive 
soil  type ;  in  other  cases  the  local  variations  may  be  so 


x.]  PURPOSE  OF  SOIL  SURVEYS  293 

much  in  evidence  that  the  "typical  soil"  can  only  be 
constructed  by  putting  together  the  results  of  many 
separate  determinations. 

But    the    practice    nf  mngfrfflcti'ng  a   typical   SOJ1   for 

analysis  by  mixing  together  equal  fractions  of  many 
samples  drawn  m  the  area  in  question,  is  not  to 
be  recommended.  Not  only  may  an  entirely  foreign 
or  accidentally  impure  sample  be  introduced  without 
detection,  but,  further,  the  limits  of  variation  normally 
to  be  expected  in  individual  soils  of  the  same  type  is 
just  as  important  as  the  composition  of  the  type  itself. 
Again,  the  existence  of  unsuspected  systematic  varia- 
tions is  entirely  obscured  by  any  process  of  mixing  \ 
samples.  The  character  of  the  information  which  \ 
should  accompany  the  soil  maps  must  largely  depend  / 
on  the  purpose  of  the  survey,  whether  it  is  concerned  \ 
with  the  agriculture  of  an  old  and  settled  country,  or 
whether  it  partakes  of  the  nature  of  an  exploration,  and  I 
aims  at  showing  the  capacities  of  the  land  for  new 
crops  and  industries.  In  the  United  States,  for  ^ 
example,  the  latter  form  of  soil  survey  is  exemplified  ; 
in  many  parts  of  the  country  agriculture  is  so  recent 
that  there  is  no  accumulation  of  experience  as  to  the 
crops  most  suited  to  each  kind  of  land ;  hence  the 
survey,  by  comparisons  of  the  texture  of  the  soil,  the 
climatic  conditions,  and  the  depth  to  ground  water, 
with  the  conditions  prevailing  in  better  known  areas, 
can  directly  tell  the  settler  with  what  crops  he  is  most 
likely  to  succeed.  The  cultivation  of  special  crops  like 
tobacco  and  sugar  beet,  to  take  two  examples  of  special 
interest  at  the  present  time  in  the  United  States, 
can  be  extended  into  new  districts  possessing  suitable 
soils,  with  a  minimum  of  the  risk  which  must  always 
attend  the  introduction  of  a  novel  form  of  culture.  The 
suitability  of  other  classes  of  land  for  irrigation,  the 


294  SOIL  TYPES  [CHAP. 

nature  and  extent  of  already  existing  alkali  patches, 
and  the  most  promising  methods  of  reclamation,  are 
also  prominent  features  in  the  work  of  the  United 
States  soil  survey.  As  the  crops  in  a  new  country  of 
this  kind  are  in  the  main  grown  by  the  aid  of  the 
natural  fertility  of  the  soil  alone,  and  fertilisers  are 

xlittle  used,  the  chemical  examination  of  the  soil  becomes 

\  of  less  importance  than  the  mechanical. 

In  a  settled  country  like  our  own,  the  character  of 
the  information  to  be  derived  from  a  soil  survey  is  of  a 
different  order  ;  the  land  has  been  under  cultivation  so 
long  that  a  great  mass  of  local  information,  based  upon 
experience,  exists  as  to  the  character  even  of  individual 
fields.  ft) 

Hints  as  to  the  cultivation,  based  upon  the  texture 
of  the  soil  as  determined  by  analysis,  would  be  too 
general  to  be  of  any  service ;  indeed,  it  is  rather  to 
be  hoped  that  by  collating  many  mechanical  analyses 
with  the  information  derived  from  men  possessing  long 
experience  of  the  soil,  further  light  can  be  shed  upon 
the  connection  between  physical  structure  and  the  finer 
points  of  tillage.  The  suitability  of  the  different  types 
[  V  of  soil  to  new  crops — as,  for  example,  the  extension  of 
the  area  under  fruit — can  be  ascertained,  and  many 
expensive  mistakes  due  to  planting  on  unsuitable  land 
could  be  saved  to  the  farmer.  Suggestions  can  also  be 
made  as  to  the  amelioration  of  the  soil  by  drainage,  or 
by  the  incorporation  of  materials  like  clay,  chalk,  or 
marl,  occurring  in  the  vicinity.  Fifty  years  ago,  no 
department  of  British  agriculture  was  more  carefully 
attended  to  than  the  improvement  of  the  texture  of 
the  soil,  and  great  tracts  of  what  is  now  fertile  land 
were  practically  created ;  lower  values  to-day  have 
caused  this  important  matter  to  be  almost  entirely 
neglected. 


X.]  APPLICATION  OF  SOIL  SURVEYS  295 

But  the  chief  application  of  a  soil  survey  in  this 
country  lies  in  the  information  that  can  be  afforded 
as  to  the  use  of  manures  ;  enormous  economies  might 
be  effected  in  the  bills  of  almost  every  farmer  using 
artificial  manures,  if  the  latter  were  properly  adapted 
to  his  soils  and  crops.  Farmers  arc  often  recom- 
mended to  carry  out  manurial  trials  upon  their  own 
farms  until  they  have  ascertained  the  peculiarities  and 
specific  requirements  of  the  soil,  but  advice  of  this 
kind  treats  altogether  too  lightly  the  somewhat  delicate 
business  of  conducting  field  experiments.  Putting  aside 
the  mechanical  difficulties  attending  a  trial  of  this  kind, 
and  the  overpowering  effect  of  minor  inequalities  of  the 
ground  and  other  accidental  conditions  which  so  often 
nullify  the  experimental  treatment,  it  is  rarely  that 
the  farmer  will  be  found  able  to  arrange  a  scheme  of 
experiment  likely  to  give  information  of  permanent 
value.  If  one  may  judge  from  the  published  accounts 
of  many  field  experiments  carried  out  in  this  country 
by  public  bodies,  which  so  often  show  a  misappre- 
hension of  the  points  really  at  issue,  there  is  every 
probability  that  the  individual  farmer  will  be  as  often 
misled  as  guided  by  the  results  of  his  own  experiments. 
The  design  and  conduct  of  field  experiments  must  be 
left  to  the  expert,  who  surveys  the  subject  from  a  wider 
standpoint,  who  can  compare  various  trials,  and  is  in  a 
position  to  continue  them  for  a  period  of  years,  rejecting 
at  any  early  stage  a  considerable  proportion  which  are 
inevitably  vitiated  by  some  concealed  local  peculiarity. 
A  body  of  experts  conducting  a  soil  survey  and  field 
experiments  simultaneously  in  the  same  area  and  co- 
ordinating their  results,  can  give  advice  of  the  most 
definite  character  as  to  the  scheme  of  manuring  to  be 
adopted  for  each  soil  type.  The  fundamental  factor 
requiring  consideration  in  this  matter,  and  brought  out 


296  SOIL  TYPES  [CHAP. 

in  the  soil  survey,  is  the  proportion  of  lime  normal  to 
each  soil  type ;  knowing  this  factor,  and  the  retentivity 
of  the  soil  for  moisture  under  ordinary  conditions  of 
rainfall,  one  can  decide  upon  the  character  of  the 
manures  for  most  loamy  soils.  Soils  of  more  specific 
character,  like  the  sands  or  clays,  present  more  character- 
istic deficiencies  of  some  of  the  manurial  constituents,  so 
that  for  many  crops  the  use  of  manures  like  phosphates 
and  potash  is  wholly  determined  by  the  soil  and  not  the 
crop. 

It  is  not  too  much  to  say  that  the  information  as 
to  the  manuring  which  is  being  accumulated  at  many 
experimental  centres  throughout  the  country  can  only 
be  rendered  properly  available  by  the  execution  of 
a  soil  survey  in  the  district  under  consideration.  In 
many  countries  a  soil  survey  has  been  made  part  of 
the  national  service  for  the  agriculturist ;  the  mag- 
nificent publications  of  the  Division  of  Soils  of  the 
United  States  Department  of  Agriculture  form  a  case 
in  point.  In  Prussia,  the  maps  and  reports  of  the 
Laboratorium  fur  Bodenkunde  at  Berlin  may  be  con- 
sulted as  models  of  the  thoroughness  and  refinement 
to  which  work  of  this  kind  can  be  pushed ;  the 
Gembloux  Station  in  Belgium  is  executing  a  system- 
atic chemical  survey  of  the  Belgian  soils.  In  France, 
the  work  rests  with  the  local  authorities  of  each 
Department,  but  in  parts  is  being  carried  out, 
as  witness  the  beautiful  maps  due  to  the  single- 
handed  work  of  M.  Gaillon,  Director  of  the  Station 
Agronomique  de  1'Aisne  at  Laon.  In  Britain  the 
great  initial  want  is  the  publication  of  drift  maps 
of  the  geological  survey  on  the  6-inch  scale ;  the 
i -inch  to  the  mile  survey,  which  alone  has  been 
published,  or  even  executed  in  most  districts,  is  too 
small  to  admit  of  necessary  detail.  It  is  also  very 


x.]  SOIL  MAPS  297 

often  laid  down  on  an  early  cadastral  survey,  which 
makes  the  identification  of  the  modern  boundaries  a 
matter  of  difficulty.  If  the  country  were  in  possession 
of  a  series  of  "  drift "  maps  on  the  scale  of  6  inches  to 
the  mile,  the  work  could  be  rapidly  supplemented  by 
soil  surveys  and  analyses  executed  by  the  local  agri- 
cultural colleges  and  research  institutions,  until  every 
farmer  could  be  put  in  possession  of  that  exact 
knowledge  of  the  soil  which  is  fundamental  to  all 
farming  operations. 


A  PPENDICES 


APPEN 


CHEMICAL   ANALYSES 
PERCENTAGES  ON  THE 


NUMBKR         .... 

1 

2 

3 

WOODNHS 

Ki 

4 

5 

6 

DISTRICT 

WISLBY,  SURREY. 

BOROUGH, 
NT. 

TKYNRAM, 

K:-  .--    . 

FORMATION  .... 

Bagshot  Sand. 

Oldhaven  Beds. 

Tbanet  Sand. 

NATURE       .       .        .       - 

Very  poor,  light 
land,  much  of  it 
in  waste. 

Very  Light  Sand, 
valuable  for  Market 
Gardening,  but  not 
for  General 
Farming. 

Light  loam  of 
great  repute  for 
fertility. 

TILTH  

Arable. 

Arable. 

Arable. 

Soil. 

Subsoil. 

Soil. 

Subsoil. 

Soil. 

Subsoil. 

Moisture  .... 

0-79 

IOI 

1-05 

2-04 

1-62 

I-7I 

Loss  on  Ignition 

3-32 

2.18 

3-33 

1-93 

3-46 

2-49 

Nitrogen  .... 

O-IO 

007 

012 

008 

0-16 

0-04 

Potash      .... 

0-31 

0.17 

o-33 

0-38 

o-35 

0-47 

Potash,  soluble  in  I   per\ 
cent.  Citric  Acid   .       .  / 

O-02 

0018 

0019 

Lime        .... 

(MO 

0-31 

0-58 

Magnesia 

0-12 

0-23 

0-26 

Alumina  .... 

0-92 

1.74 

2-34 

Ferric  Oxide     . 

o-57 

1-38 

2-10 

Oxide  of  Manganese 

0-04 

0-05 

... 

0-14 

Calcium  Carbonate  . 

O-OI 

0-04 

002 

O-OI 

o-33 

0-05 

Phosphoric  Acid       . 

005 

003 

006 

0-05 

O'lO 

0-07 

Phosphoric  Acid,  soluble] 
in    I    per  cent.   Citric  ]• 
Add                               } 

OOI2 

0-017 

0044 

Sulphuric  Acid 

003 

0-05 

O-OI 

DIX    I. 


OF  TYPICAL   SOILS. 

FINB  EARTH— AIR-DRIED. 


7 

8 

9 

10 

u        » 

13              H 

IS 

16 

BUTTON  BY 
DOVKR,  KENT. 

liENTLEY, 

HANTS. 

MABDEN, 
KENT. 

WASBOHOI  on, 
SURREY. 

WOODCHURCU, 
KENT. 

Chalk. 

Upper  Greensand. 

Alluvial. 

London  Clay. 

Weald  Clay. 

Light  loam, 
"  sheep  and 

Qood  loaiu,              Heavy  loam, 
noted                   rather  poor. 

Heavy  loam, 
of  fair  repute. 

Very  heavy  clay, 
of  little  value. 

barley  land." 

for  fertility. 

Arable. 

Arable. 

Arable. 

Arable. 

Arable. 

Soil. 

Subsoil. 

Soil. 

Subsoil. 

Soil. 

Subsoil. 

Soil.       Subsoil. 

Boil. 

Subsoil. 

6-76 

4-18 

3-23 

4-22 

2-40 

3-16        3-91        2'94        4-°7 

3-62 

9-28 

7-37        4-6o 

4-03 

6-1  1 

3-78     i   4-38        3-88 

8-73 

5-37 

0-2$ 

013        0-19 

0-12 

0-18           ...          0-19        006 

0-26 

o-ia 

o-43 

0-60 

O-6o 

0-63 

0-90          ...         0-33        0-67 

103 

i-n 

0-018 

0-025 

0-031 

0-006 

0-017 

0-012 

2-61 

2-61 

0-99 

O-7O 

0-69 

0-38 

0-48 

0-37           ...         o-35 

0-31 

6-45 

9-8? 

'   8-44            •••          4-14 

6-45 

4-27 

2-25 

2-o6 

3-92           ...         >47 

8-81 

0-16 

0-12 

O-l6 

0-03 

0-71 

0-08 

18-1 

1  1  -4        0-47 

3-48 

0-64 

0-18        0-06   i     0-08 

0-08        0-03 

0-19 

0-17 

0-27 

0-15 

I    0-12 

O-O6            O-O6     ;        O-O3 

0-11 

005 

O-OOI 

0-16 

o-oS 

OOO9 

0-014 

0-004 

0-09 

0-06 

0-12 

O-O6               ...              O-O4 

0-08 

801 


APPENDIX     II 

BIBLIOGRAPHY 

The  following  short  list  of  references  will  take  the  student  to 
some  of  the  more  important  original  sources  of  information  on 
each  of  the  subjects  treated  of  in  this  book.  They  have  been 
selected  from  among  the  great  mass  of  papers  that  exist,  not 
because  they  are  necessarily  the  most  important,  but  as  suggestive 
in  themselves,  and  likely  to  lead  the  student  to  make  further 
acquaintance  with  the  methods  as  well  as  with  the  results  of 
research.  Several  of  the  papers  also  contain  a  series  of  references 
to  other  workers  in  the  same  field. 

THE  ORIGIN  OF  SOILS. 

1.  RlSLER,  E. — Geologic  Agricole,  4  vols.,  Paris,  1884-97. 

2.  MERRILL,  G.  P. — Rocks,  Rock   Weathering,  and  Soils,  New 

York,  1897. 

ANALYSIS  AND  COMPOSITION  OF  SOILS. 

3.  LAUNFER  AND  WAHNSCHAFFE. — Mit.  a.  d.  Laboratorium  fur 

Bodenkunde,  B.  III.,  h.  2,  Berlin. 

4.  PETERMANN. — Recherches  de  Chimie  le,  III.,  Brussels,  1898. 

5.  HILGARD,  U.S.  Dep.  of  Agric.,  Div.  of  Chem.,  Bull.  38,  1893. 

6.  HILGARD,  U.S.  Dep.  of  Agric.,  Div.  of  Soils,  Bull.  4,  1896. 

7.  HILGARD. — Soils,  The  Macmillan  Company,  New  York,  1906. 

8.  MITSCHERLICH,  E.  A. — Bodenkunde  fiir  Land  u.  Forstwirte, 

1905. 

9.  LAWES  AND  GILBERT. — Rothamsted  Memoirs,  V.,  No.  19. 
10.  DYER.—/.  Chem.  Soc.,  1894,  65,  115. 

u.  DYER.—/3////.  Trans.,  194,  B.  (1901),  235. 

12.  DYER. — U.S.  Dep.  of  Agric.,  Bull.  106,  1902. 

13.  HALL,  PLYMEN,  AND  AMOS. — Trans.   Chem.   Soc.,  1902,  81, 

117;  1906,  89,  207. 

14.  HALL. — "Analysis  of  Soil  by  the  Plant,"/-  Agric.  Sci.,  1905, 

1,65. 

808 


APPENDIX  11  303 

Son.  PHYSICS. 

15.  VI  M.\WIQK.— Physical  Properties  of  Soils,  Oxford,  1900. 

16.  KING. — Physics  of  Agriculture,  Madison,  VVis.,  1901. 

17.  HlLGARD.— U.S.   Dep.   of  Agric.,  Weather   Bureau,    Bull.   3, 

1892. 

18.  WHITNEY.— U.S.  Dep.  of  Agric.,  Weather  Bureau,  Bull.  4, 

1892. 

19.  HELLRIEGEL. — B.  z.    d.    nat.    Grundlagen  des   Ackerbaues, 

Braunschweig,  1883. 

20.  LAWES  AND  GILBERT. — Rothamsted  Memoirs,  Vol.  III.,  No. 

1 1  ;  Vol.  V.,  No.  5. 

21.  BRIGGS. — U.S.  Dep.  of  Agric.  Year- Book,  1900,  397. 
See  also  Hilgard,  No.  7  ;  Wollny,  No.  20. 

SOIL  ORGANISMS. 

22.  WOLLNY. — Zersetzung  der    Organise/ten    Stoffe,    Heidelberg, 

1897. 

23.  OMELIANSKY. — Compt.  Rend.,   121   (1895),  653;    125  (1897), 

970. 

24.  WlNOGRADSKY. — Compt.  Rend.,  121  (1895),  742. 

25.  LAFAR. — Technische  Mycologie,  2nd  ed.,  Fischer,  Jena,  1904. 

FIXATION  OF  NITROGEN. 

26.  LAWES  AND  GILBERT.— Phil.  Trans.,  II.,  431,  1861  ;    15.   I., 

1889. 

27.  LAWES  AND  GILBERT.—/.  R.  Ag.  Soc.,  E.,  3rd  s.,  II.,  657, 

1891. 

28.  HELLRIEGEL  AND  WILFARTH.— D.  Land.    Vers.  Stat.,  460, 

1887. 

29.  NOBBE  AND   HlLTNER. — D.  iMtld.  Vers.  Stat.,  1899. 

30.  WlNOGRADSKY. — Compt.  Rend.,  118  (1894). 

31.  MAZE. — Ann.  de  I'lnst.  Pasteur,  u  (1897). 

32.  JACOBITZ.— Cent,  fiir  Bakt.,  II.,  1901,  7,  783. 

33.  BEIJERINCK.— Cent.fitrBakt.,  II.,  1902,9,  3. 

NITRIFICATION. 

34.  SCHLOKSING  AND  Mi'NTZ.  —  Compt.  Rend.,  80  ( 1877),  1250. 

35.  WARINGTON.— /.  C.  S.,  1878,  1879,  1884,  1891. 

36.  LAWES,  GILBERT,  AND  WARINGTON.— /.   A'.  Ag.   Soc.,  E., 

2nd  s.,  19  (1883). 


304  APPENDIX  II 

37.  WINOGRADSKY.— Compt.  Rend.,  113  (1893),  116. 

38.  DEH£RAIN. — Ann.  Agron.,  19,  409. 

39.  KING. — Wisconsin    Ag.   Exp.    Station    Annual    Reports,    17 

(1900),  18  (1901). 

40.  BOULLANGER  AND  MASSOL. — Ann.  Pasteur,  1903,  17,  492. 

DENITRIFICATION. 

41.  GAYON  AND  DUPETIT.— Compt.  Rend.,  95  (1882),  644. 

42.  WAGNER. — D.  Landiv.  Presse,  1895,  123. 

43.  WARINGTON.— /.  /?.  Ag.  Soc.,  E.,  yd  s.,  8. 

44.  STOKLASA.— Cent.f.  Bakt.,  7  (1901),  260. 

MYCORHIZA. 

45.  Sl&H'L.—Jahrb.f.  iviss.  Botanik.,  34. 

ABSORPTION  OF  SALTS  BY  SOIL. 

46.  WAY.—/.  A'.  Ag.  Soc.,  E.,  ist  s.,  n,  323  ;  13,  123. 

47.  VOELCKER.— /.  R.  Ag.  See.,  £.,   ist   s.,  21    (1860),   93;    25 

(1864),  333- 

48.  VOELCKER.—/.  R.  Ag.  Soc.,  E.,  2nd  s.,  10  (1874),  132. 

49.  LAWES,    GILBERT,  AND  WARINGTON.—/.  R.  Ag.  Soc.,  E., 

2nd  s.,  XVII.  (1881),  241  ;  XVIII.  (1882),  i. 

50.  HALL  AND  GIMINGHAM,  Trans.  Chem.  Soc.,  91  (1907),  677. 

ALKALI  SOILS. 

51.  HlLGARD. — U'.S.  Dep.  of  Agric.  Year-Book,  1895,  103. 

52.  HlLGARD. — Univ.  of  California  Ag.  Exp.  Sta.  Bull.  128  (1900). 

53.  WiLLCOCKS. — Egyptian  Irrigation,  2nd  ed. 

SOIL  SURVEYS,  ETC. 

54.  SCHIMPER,  A.  F.  VI .—Pflanzen  Geographic,  Jena,  1898. 

55.  U.S.  Dep.  of  Agric.,  Div.  of  Soils,  "Field  Operations,"  1901. 

56.  WHITNEY.— "  Tobacco  Soils,"  U.S.  Dep.  of  Agric.  Bull,  n, 

1898. 

57.  VEITCH.— "  Maryland  Soils,"   Maryland  Ag.  Exp.  Sta.  Bull. 

70,  1901. 
See  also  Nos.  3  and  4. 


INDEX 


ABSORPTION  of  salts  by  soil,  an. 

Acid  soils,  44,  204,  242. 

Aerobic  bacteria,  171. 

Alinit,  174. 

Alkali  soils,  245,  272. 

Alluvial  soils,  279. 

Alluvium,  12. 

Altitude,  effect  of  temperature,  133. 

Alway,  soil-water  required  for  crops, 
86. 

Ammonia,  absorption  by  soil,  214; 
formation  from  urea,  172  ;  salts, 
removal  of  lime  by,  216,  224. 

Anaerobic  bacteria,  174. 

Analysis,  available  constituents,  158  ; 
chemical,  139;  interpretation  of, 
151,  165  ;  mechanical,  32,  50;  of 
soil  by  plant,  155. 

Anbury  in  turnips,  204,  208. 

Angus  Smith  on  denitrification,  197. 

Anticyclones,  reversal  of  tempera- 
ture in,  134. 

Apatite,  24. 

Arid  soils,  characteristics  of,  46. 

Assimilation  and  transpiration,  90. 

Atwater  on  fixation  of  nitrogen,  179. 

Augite,  21. 

Autumn  cultivation,  gain  of  water 
by,  95- 

Available  plant  food  in  the  soil,  157, 
164. 

Azotobacter  chroococcum,  187. 
805 


BACTERIA  in  soil,  4,  168  ;  denitri- 
fying, 197  ;  nitrifying,  191  ;  iron, 
203  ;  nitrogen  fixing,  174. 

Bailey-Denton  on  effect  of  drainage 
on  temperature,  132. 

Bare  fallows,  86,  112. 

Basalt,  weathering  of,  21. 

Beaumont,  E.  de,  on  antiquity  of 
soil,  29. 

Beijerinck  on  Azotobacter,  187. 

Benetzungs-warme,  88. 

Berthelot  on  fixation  of  nitrogen, 
179,  1 86  ;  and  Andre  on  nitrogen 
in  humus,  47. 

Black  soils,  235. 

Bog  iron  ore,  25. 

Boussingault  on  source  of  nitrogen 
in  plants,  175,  177. 

Brick  earth,  13,  15. 

Brown  on  energy  absorbed  in 
assimilation,  90. 

Brownian  motion,  38. 


CALCAREOUS  soils,  281. 

Calcifuges,  276,  288. 

Calcium    carbonate,    22,    40 ;    aids 

decay    of    organic    matter,    173  ; 

determination    of,    144 ;    in    soils 

subject  to  "  finger-and-toe,"  209  ; 

removal  by  manures,  2 1 6. 
Capacity  of  soil  for  water,  67. 
U 


3o6 


INDEX 


Capillarity,  72  ;  supply  of  water  to 
crop  by,  97,  106. 

Carbon,  determination  of,  143. 

Carbon  to  nitrogen  in  soils,  ratio 
of,  46. 

Carbonic  acid  in  soil  gases,  13  ; 
excreted  by  roots,  158  ;  and  water, 
action  on  rock  minerals,  1 3. 

Catch  crops,  IIO. 

Chalk,  41;  "pipes"  in,  due  to 
weathering,  23  ;  soils,  281. 

Chalking,  261. 

Chemical  analysis  of  soil,  139. 

China  clay,  19,  35. 

Chlorosis,  283. 

Citric  acid  as  solvent  in  soil  analysis, 
159- 

Clay,  colloid,  35  ;  flocculation  of, 
38  ;  impermeability  to  water,  34, 
71  ;  nature  of,  36,  52  ;  origin  of, 
19  ;  shrinkage  on  drying,  34,  284  ; 
soils,  30,  283. 

Claying,  256. 

Cleopatra's  Needle,  weathering  of, 
ii. 

Climate  and  situation,  136. 

Clostridium  Pastorianum,  187. 

Club  root  in  turnips,  204,  208. 

Cohesion  due  to  surface  tension,  82. 

Colloid  clay,  35. 

Colmetage,  255. 

Colour  of  soils,  27;  and  temperature, 
127. 

Condensation  of  water  by  soil,  87. 

Condition  of  land,  233. 

Conventions  necessary  in  chemical 
analysis  of  soil,  141. 

Crops,  plant  food  removed  by,  152. 

Crops,  water  required  by,  91,  loS. 

Cultivation,  effect  of,  on  water  con- 
tent of  soil,  95,  101,  104. 

DARWIN  on   action  of  earthworms, 

14. 
Daubree  on  size  of  sand  grains  not 


rounded  by  running  water,  1 7  ; 
weathering  of  felspar,  19. 

Deflocculation,  39,  252. 

Defoe,  definition  of  manure,  1 54. 

Dehe"rain  on  available  phosphoric 
acid  in  soils,  158  ;  nitrates  in 
drainage  waters,  228 ;  nitrifica- 
tion, 195. 

Deherain  and  Maquenne  on  denitri- 
fication,  198. 

Denitrification,  197. 

Density  of  soils,  63. 

Drainage,  93  ;  required  with  irriga- 
tion, 249;  warms  the  land,  132; 
waters,  composition  of,  222. 

Drains,  flow  of,  77. 

Drain-gauges,  78  ;  flow  greater  than 
rainfall,  87. 

Drift,  glacial,  15  ;  maps,  297;  soils,  8. 

Drought  of  1870  at  Rothamsted, 
108. 

Drought,  susceptibility  of  different 
soils  to,  99. 

Drying  effect  of  crops,  86,  108. 

Dunbar,  potato  soils  of,  I. 

Dung,  effects  on  water  in  soil,  117; 
and  denitrification,  201. 

Dyer  on  determination  of  available 
phosphoric  acid  and  potash,  159; 
potash  and  phosphoric  acid  re- 
tained by  Rothamsted  soils,  2 1 8. 

EARLY  and  late  soils,  135. 
Ebelmar  on  weathering  of  basalt,  21. 
Egypt,  irrigation  in,  248. 
Eremacausis,  170. 
Evaporation,  cooling  effect  of,  130  ; 

losses  of  water  by,  97. 
Exhausting  effect,  of  nitrate  of  soda, 

253  ;  of  wheat,  237. 

FAIRY  rings,  239. 
Fallows,  bare,  86,  112. 
Felspar,  1 8. 


INDEX 


3«>7 


Fen  land,  potatoes  from  the  black 

soils  of,  2  ;  reclamation  of,  258. 
Ferrous  carbonate  in  soils,  145. 
Fertility,  233. 
Film    of     water     surrounding    soil 

particles,  73. 
"  Finger-and-toe,"    204,    208,    243, 

276. 

Fixation  of  nitrogen,  174- 
Flint,  18. 

Flocculation  of  clay,  38,  40,  253. 
Flow  of  water  through  soils,  70. 
Frank  on  mycorhiza,  205. 
Frosts  in  valleys,  135  ;  killing  effect 

of,  124. 

Fruit  trees  in  grass  land,  III. 
Fuller's  earth,  37. 
Fungi  in  the  soil,  168,  204. 


GAYON  and  Dupetit  on  denitrifica- 

tion,  198. 

Germination,  temperatures  of,  124. 
Glacial  drift,  1 5. 
Glauconite,  25. 
Granite,  weathering  of,  19. 
Gravel,  15. 
Green  manuring,  185. 
Greenstone,  weathering  of,  21. 
Grey  wethers,  17. 
Gypsum,  24. 


HALOPHYTES,  280. 

Hanamann  on  weathering  of  basalt, 

22. 
Heat  received  by  the  soil,  120,  126  ; 

required  for  evaporation,  150. 
Heavy  soils,  64. 
Ileinrich  on   hygroscopic   moisture, 

85- 

Hellriegel  on  fixation  of  nitrogen, 
176,  179  ;  on  optimum  proportion 
of  water  for  growth,  69  ;  on  «;Uer 
transpired  by  crops,  89. 


Hilgard  on  alkali  soils,  246  ;  on  dis- 
tillation of  water  from  subsoil,  88 ; 
on  mechanical  analysis  of  soils, 
50  ;  on  method  for  determination 
of  water  capacity,  67. 

Hilgard  and  Jaffa  on  nitrogen  in 
humus,  46. 

Hiltncr,  cultivation  of  nodule  organ- 
isms, 183. 

Hoeing,  102. 

Hornblende,  21. 

Humic  acid,  44. 

Humus,  24,  27,  42,   118,  168,  171, 

174,  243. 

Humus,  soluble,  44,  165. 
Hygroscopic  moisture,  84. 

IGNITION,  loss  on,  143. 

Inoculation  of  soil,  183. 
Interpretation  of  soil  analysis,  151, 

!65. 

Irrigation,  246. 
Iron  bacteria,  203. 
Iron  pan  in  soils,  24,  27Si  285. 
Iron  pyrites,   26  ;  causing   sterility, 
243- 

KAOLINITE,  19,  34,  36. 

King  on  area  of  surface  of  soil 
particles,  66 ;  on  bare  fallows, 
113;  on  effect  of  cultivation  on 
water  content  of  soil,  96,  101,  103; 
on  effect  of  slope  on  temperature 
of  soil,  133 ;  on  flow  of  water 
through  sand,  70  ;  on  nitrates  in 
soil,  195  ;  on  percolation,  76;  on 
transpiration  water,  89  ;  on  water 
in  soils  too  dry  for  growth.  85. 

Kossowitsch  on  fixation  of  nitrogen, 
lS6. 

Kriiger  and  Schnei<Je\Miid,  186,  200. 

LAKING,  213. 

Langlcy  on  solar  radi.itkm,  126. 


308 


INDEX 


Late  soils,  135. 

Laurent  on  fixation  of  nitrogen, 
III. 

Lawes  and  Gilbert  on  fixation  of 
nitrogen,  176  ;  on  ratio  of  carbon 
to  nitrogen  in  soils,  46  ;  on  tran- 
spiration water,  89.  (See  also 
Rothamsted.) 

Leguminous  plants,  fixation  of 
nitrogen  by,  1 80  ;  green  manur- 
ing with,  185. 

Liebig,  mineral  theory,  175. 

Light  soils,  64. 

Lime,  effect  on  temperature  of  soil, 
127;  composition  of,  268  ;  floccu- 
lation  of  clay  by,  40;  for  "  finger- 
and-toe,"  209  ;  in  drainage  waters, 
223  ;  removal  from  soil  by  use  of 
ammonium  salts,  216,  224. 

Liming,  41,  209,  261. 

Limonite,  24. 

Loams,  31,  278. 

Loess  deposits,  10. 

Lois-Weedon  husbandry,  114. 

Lucerne  requiring  inoculation,  184. 

Lysimeter,  78. 


MAGNESIA  in  drainage  waters,  225  ; 
in  soils,  146,  243,  285. 

Manures,  absorption  of,  by  soil,  2 1 1 ; 
effect  on  texture  of  soil,  251; 
original  signification  of,  1 54  ;  re- 
covery of,  in  crop,  164 ;  time  of 
application  of,  229. 

Marcasite,  26. 

Marling,  256. 

Marls,  31,  257. 

Mechanical  analysis  of  soil,  32,  50. 

Mica,  20. 

Mineral  theory,  Liebig's,  175. 

Minerals,  composition  of  rock-form- 
ing, 1 6. 

Mitscherlich  on  benetzungs-warme, 
88. 


Molecular  forces,  71. 
Molisch  on  iron  bacteria,  203. 
Moor-band  pan,  25. 
Moore,  cultivation  of  nodule  organ- 
isms, 183. 
Mould,  171. 
Mulches,  102,  246. 
Miiller  on  nitrification,  191. 
Mycorhiza,  205,  277. 


NATROLITE,  26. 

Nile  water,  256. 

Nitragin,  183. 

Nitrates,  determination  of,  147;  for- 
mation of,  112,  194;  in  drainage 
waters,  226 ;  in  soil,  effect  of 
cultivation  on,  195  ;  produced  by 
bare  fallow,  112,  196. 

Nitrification,  191. 

Nitrogen,  determination  of,  144 ; 
fixation  of,  174 ;  proportion  re- 
covered in  crop,  199,  202. 

Nobbe  on  nitragin,  183. 

Nodules  on  leguminous  plants,  180. 

OEMLER  on  specific  heat  of  soil, 
129. 

Olivine,  22. 

Optimum  proportion  of  water  in 
soils,  for  growth,  69 ;  tempera- 
tures for  growth,  123. 

Orchids,  germination  of,  208. 

Osborne  on  mechanical  analysis  of 
soils,  50. 

Osmosis,  entry  of  plant  food  by, 
157- 


PACKING  of  soil  particles,  6r. 
Pan,  iron,  in  soils,  24,  275,  285. 
Paring  and  burning,  259. 
Parkes  on  temperatures  of  drained 
and  undrained  land,  132. 


INDEX 


309 


Peat,  origin  of,  43,  174  ;  reclamation 

of,  258  ;  soils,  288. 
Percolation,     75,     115;      through 

gauges  at  Rothamsted,  78. 
Phosphoric   acid,   determination   of, 

146  ;  retention  by  soil,  219. 
Pipe  clays,  35. 
Pipes  in  chalk  rock,  23. 
Plasmodiophora  brassicae,  204,  208. 
Plasticity  of  clay,  34,  40. 
Ploughing,  damage  done  by  deep, 

29,  285. 
Pore  space,  60. 
Potash,  determination  of,  146,  150; 

origin  in  soils,  19,  28  ;  retention 

by  soil,  217. 
Potatoes   grown   on  different   soils, 

value  of,  I. 
Pseudomonas  radicicola,  180,  182. 


Qi'ALITY  of  produce  from  early  and 

late  soils,  137. 
Quarry  water,  II. 
Quartz,  17. 
Quincke    on    range    of    molecular 

forces,  71. 


Radiation,  126. 

Rainfall  and  crops,  91. 

Rainfall  and  drainage,  115. 

Recovery  of  manures  in  crop,  164. 

Red  lands  of  Dunbur,  value  of 
potatoes  from,  I. 

Richthoven  on  loess  deposits,  10. 

Rock-forming  minerals,  16. 

Rolling,  effect  of,  105. 

Roots,  solvent  action  of,  158  ; 
weathering  action  of,  14. 

Rothamsted,  available  constituents 
in  soil  of,  162  ;  bare  fallows  at, 
114;  composition  of  drainage 
waters  at,  222  ;  composition  of 
soil,  37,  58  ;  drain  gauges  at,  78  ; 


drought  of  1870  at,  108  ;  investi- 
gations on  fixation  of  nitrogen  at, 
176, 178,  189  ;  lime  in  soil  of,  2 1 6; 
losses  of  nitrogen  in  manure  at, 
201;  method  of  sampling  soils  at, 
48  ;  nitrates  in  soil  at,  196  ;  phos- 
phoric acid  in  soil  of,  162,  22 1; 
potash  in  soil  of,  21 8  ;  weights  of 
soil  at,  64. 
Rutile,  34. 


SACHS  on  solvent  action  of  roots, 
158;  on  temperature  and  tran- 
spiration, 124  ;  on  water  in  soil, 
when  plants  wilt,  84. 

Salfeld  on  soil  inoculation,  185. 

Salt,  causing  sterility,  244  ;  marshes, 
vegetation  of,  279. 

Salts,  soluble  in  soils,  147. 

Sampling  of  soils,  47. 

Sand,  origin  of,  17,  33  ;  velocity  of 
current  required  to  carry,  33. 

Sandy  soils,  30,  242,  274. 

Saturation  of  soils  by  water,  69,  77. 

Saussure  on  theory  of  nutrition,  175. 

Schloesing  on  colloid  clay,  35  ;  on 
mechanical  analysis  of  soils,  50. 

Schloesing  and  Muntz  on  nitrifica- 
tion, 191. 

Schloesing^Y*  on  fixation  of  nitrogen, 
181. 

Schneidewind  on  denitrification,  200; 
on  fixation  of  nitrogen,  1 86. 

Schultz  on  green  manuring,  185. 

Sea,  effect  on  climate,  136. 

Sedentary  soils,  7. 

Selenite,  24,  26,  285. 

Senft  on  decomposition  of  felspar, 
19. 

Serpentine.  22. 

Sewage  farms,  crops  on,  92  ;  purifi- 
cation of,  by  soil,  213. 

Shingle,  growth   of  vegetation   on, 

10. 


INDEX 


Shrinkage  of  clay  on  drying,  34, 
284. 

Silica,  17. 

Silt,  53- 

Soil,  alkali,  245,  272  ;  analyses  of 
typical,  Appendix  I.;  description 
of,  7,  29;  drift,  8;  gases,  13; 
inoculation,  183 ;  mulch,  107;  of 
transport,  8  ;  relation  to  subsoil, 
7,  26,  194  ;  sedentary,  7;  surveys, 
289;  temperatures,  122  ;  typical, 
55,  149,  271;  worn-out,  28,  257. 

Soot,  effect  on  temperature  of  soil, 
128  ;  effect  on  texture  of  soil,  253. 

Specific  heat  of  soils,  128,  275. 

Spring  on  impermeability  of  clay, 

71- 
Spring  cultivation,  gain  of  water  by, 

97;  frosts,  135. 
Stahl  on  mycorhiza,  205. 
Sterility  of  soils,  241. 
Stoklasa  on  alinit,  173. 
Stones,   reputed   growth   on   arable 

land,   1 2  ;    retention  of  moisture 

by,  105  ;  warming  effect  of,  131. 
Subsoil,  description  of,  7,  26,  194  ; 

as  regulator  of  water  supply,  100  ; 

packing,  107. 
Surface  of  soil  particles,  area  of,  65  ; 

rise  of  salts  to  the,  245. 
Surface  tension,  71;    cohesion   due 

to,  82  ;  variations  in,   8 1  ;  water 

lifted  by,  97,  106. 
Surveys,  soil,  289. 
Symbiosis,  169. 


TALC,  22. 

Temperature,    effect   of    colour   on, 

127;  effect  of  exposure  on,  133  ; 

required   for    growth,    120,    123  ; 

weathering    due     to    alternations 

of,  9. 

Texture  of  soil,  32,  60,  251. 
Thanet  sand,  soils  on,  236. 


Tillage  and  soil  water,  89. 
Tilth,  97,  253. 
Transpiration  water,  89. 
Typical  soils,  chemical  analysis  of, 
149  ;  mechanical  analysis  of,  55. 

UREA,    conversion    into    ammonia, 
172. 


VALLEY  frosts,  1 34. 

Velocity  of  currents  carrying  sand 
grains,  33. 

Ville  on  fixation  of  nitrogen,  179. 

Vineyards,  stones  on  surface  of,  105. 

Virgin  soils,  nitrogen  in,  189. 

Voelcker  on  absorption  of  manures 
by  soil,  212  ;  on  analysis  of  drain- 
age waters,  222  ;  on  fixation  of 
nitrogen,  179;  on  "finger-and- 
toe"  in  turnips,  209. 


WAGNER  on  denitrification,  199. 

Warington  on  determinations  of 
nitrates  in  soils,  147;  on  denitri- 
fication, 198  ;  on  nitrification,  191; 
on  wetness  of  land  in  February, 
88. 

Warp  soils,  255  ;  growth  of  potatoes 
on,  2. 

Water  capacity  of  soil,  67. 

Water  table,  77. 

Way  on  absorption  of  salts  by  soil, 
212. 

Weathering,  7,  8  ;  due  to  alterna- 
tions of  temperature,  9  ;  due  to 
frost,  1 1 ;  due  to  water  and  car- 
bonic acid,  13  ;  due  to  roots,  14  ; 
of  granite,  19  ;  of  basalt,  21. 

Weight  of  a  cubic  foot  of  soil,  64. 

Wheat  as  an  exhausting  crop,  237. 

Wilfarth  on  fixation  of  nitrogen,  1 80. 

Wilting  of  plants  in  soils  containing 
water,  84. 


INDEX 


Wind,  transport  of  soil  by,  to. 

Winogradsky  on  iron  bacteria,  203, 
286 ;  on  nitrification,  192  ;  on 
nitrogen-fixing  bacteria,  1 86. 

Woburn,  lime  in  soil  at,  204  ; 
residues  left  by  ammonium  salts 
at,  232  ;  weight  of  cubic  foot  of 
soil  at,  64  ;  fruit  farm,  growth  of 
trees  planted  with  grass,  112. 

Wollny  on  composition  of  soil  leases, 
13  ;  on  effect  of  slope  on  tempera- 
ture, 133  ;  on  oxidation  of  humic 


acid,  173;  on  water  required  by 

crops,  89. 
Worms,  fine  soil  brought  to  surface 

by,  14. 
Worn-out  soils,  28,  257. 

XEROPHYTBS,  272, 277,  288. 
YEASTS  in  the  soil,  170,  204. 

ZEOLITES,  25,  39,  214. 
Zircon,  34. 


PK1NTCU    BY 
lVkR   AND   BOYb 
EDINBURGH 


UNIVERSITY  OF  CALIFORNIA  AT  LOS  ANGELES 

THE  UNIVERSITY  LIBRARY 
This  book  is  DUE  on  the  last  date  stamped  below 


Form  L-!> 

lc>in-3,'3«(77.r>2> 


r  OF  CALIFORNIA 


LOS  a] 

..•  '.••:,/-  /,  , 


s 

$91 

Hllis 

1908 


A     001  127  226     7 


